diff --git a/examples/blocking_api.c b/examples/blocking_api.c
index 75bba12..d5fc237 100644
--- a/examples/blocking_api.c
+++ b/examples/blocking_api.c
@@ -14,7 +14,7 @@ int main(void)
 
     ToastyString path = TOASTY_STR("/first_file");
 
-    int ret = toasty_create_file(toasty, path, 1024);
+    int ret = toasty_create_file(toasty, path, 1024, NULL);
     if (ret < 0) {
         printf("Couldn't create file\n");
         toasty_disconnect(toasty);
@@ -22,7 +22,7 @@ int main(void)
     }
 
     char data[] = "Hello, world!";
-    ret = toasty_write(toasty, path, 0, data, sizeof(data)-1);
+    ret = toasty_write(toasty, path, 0, data, sizeof(data)-1, 0);
     if (ret < 0) {
         printf("Couldn't write to file\n");
         toasty_disconnect(toasty);
diff --git a/inc/ToastyFS.h b/inc/ToastyFS.h
index db26fc6..ceb587d 100644
--- a/inc/ToastyFS.h
+++ b/inc/ToastyFS.h
@@ -50,19 +50,37 @@ int toasty_wakeup(ToastyFS *toasty);
 // BLOCKING API
 //////////////////////////////////////////////////////////////////////////////////
 
+// TODO: comment
+typedef uint64_t ToastyVersionTag;
+
+// TODO: comment
+#define TOASTY_VERSION_TAG_EMPTY ((ToastyVersionTag) 0)
+
 // Creates a directory at the specified path.
 // Returns 0 on success, -1 on error.
-int toasty_create_dir(ToastyFS *toasty, ToastyString path);
+//
+// If the version tag is not NULL, it's used to
+// return the version tag associated to the newly
+// created file.
+int toasty_create_dir(ToastyFS *toasty, ToastyString path,
+    ToastyVersionTag *vtag);
 
 // Creates a file with the given chunk size at
 // the specified path. Returns 0 on success, -1
 // on error. The chunk size can't be 0.
+//
+// If the version tag is not NULL, it's used to
+// return the version tag associated to the newly
+// created file.
 int toasty_create_file(ToastyFS *toasty, ToastyString path,
-    unsigned int chunk_size);
+    unsigned int chunk_size, ToastyVersionTag *vtag);
 
 // Deletes a file or directory at the specified path.
 // Returns 0 on success, -1 on error.
-int toasty_delete(ToastyFS *toasty, ToastyString path);
+//
+// If the version tag is not 0, the file/directory
+// is only deleted if the tags match.
+int toasty_delete(ToastyFS *toasty, ToastyString path, ToastyVersionTag vtag);
 
 typedef struct {
     char name[128]; // TODO: Implement a proper name length
@@ -79,8 +97,13 @@ typedef struct {
 // on success, returns -1 on error. The listing is
 // a dynamic array that needs to be freed using
 // "toasy_free_listing".
+//
+// If the version tag is not 0, the listing only
+// succedes if the tag matches the remote one.
+// If the operation succedes, the vtag is set to
+// the remote tag.
 int toasty_list(ToastyFS *toasty, ToastyString path,
-    ToastyListing *listing);
+    ToastyListing *listing, ToastyVersionTag *vtag);
 
 // Frees a listing created by "toasty_list".
 void toasty_free_listing(ToastyListing *listing);
@@ -88,14 +111,21 @@ void toasty_free_listing(ToastyListing *listing);
 // Reads "len" bytes at offset "off" from the file at
 // the given path. Returns the number of bytes read on
 // success, or -1 on error.
+//
+// If vtag is not NULL, the read only succedes if the
+// target version tag matches vtag or vtag was 0. If
+// the operation succedes, the version tag is set to
+// the remote entity's.
 int toasty_read(ToastyFS *toasty, ToastyString path, int off,
-    void *dst, int len);
+    void *dst, int len, ToastyVersionTag *vtag);
 
 // Writes "len" bytes at offset "off" to the file at
 // the given path. Returns the number of bytes written
 // on success, or -1 on error.
+//
+// For how vtag works, see toasty_read.
 int toasty_write(ToastyFS *toasty, ToastyString path, int off,
-    void *src, int len);
+    void *src, int len, ToastyVersionTag *vtag);
 
 //////////////////////////////////////////////////////////////////////////////////
 // ASYNCHRONOUS API
@@ -119,22 +149,35 @@ ToastyHandle toasty_begin_create_file(ToastyFS *toasty, ToastyString path,
 // Begins a file or directory deletion operation and
 // returns a handle to it. On error, TOASTY_INVALID is
 // returned.
-ToastyHandle toasty_begin_delete(ToastyFS *toasty, ToastyString path);
+//
+// If vtag is not 0, the operation only succedes if the
+// tag matches the remote entity's.
+ToastyHandle toasty_begin_delete(ToastyFS *toasty, ToastyString path,
+    ToastyVersionTag vtag);
 
 // Begins a directory listing operation and returns
 // a handle to it. On error, TOASTY_INVALID is returned.
-ToastyHandle toasty_begin_list(ToastyFS *toasty, ToastyString path);
+//
+// If vtag is not 0, the operation only succedes if the
+// tag matches the remote entity's.
+ToastyHandle toasty_begin_list(ToastyFS *toasty, ToastyString path, ToastyVersionTag vtag);
 
 // Begins a read operation and returns a handle to it.
 // On error, TOASTY_INVALID is returned.
+//
+// If vtag is not 0, the operation only succedes if the
+// tag matches the remote entity's.
 ToastyHandle toasty_begin_read(ToastyFS *toasty, ToastyString path,
-    int off, void *dst, int len);
+    int off, void *dst, int len, ToastyVersionTag vtag);
 
 // Begins a write operation and returns a handle to it.
 // On error, TOASTY_INVALID is returned. Note that the source
 // buffer must be valid until the operation completes.
+//
+// If vtag is not 0, the operation only succedes if the
+// tag matches the remote entity's.
 ToastyHandle toasty_begin_write(ToastyFS *toasty, ToastyString path,
-    int off, void *src, int len);
+    int off, void *src, int len, ToastyVersionTag vtag);
 
 // Associate the pointer "user" to the handle. The user
 // pointer will be returned in the ToastyResult when the
@@ -158,8 +201,9 @@ typedef enum {
 typedef struct {
     ToastyResultType type;
     ToastyListing    listing;
-    void *user;
-    int bytes_read; // For read operations: actual number of bytes read
+    ToastyVersionTag vtag;
+    void*            user;
+    int              bytes_read; // For read operations: actual number of bytes read
 } ToastyResult;
 
 // If the operation specified by "handle" is complete,
diff --git a/misc/TODO.txt b/misc/TODO.txt
index c064cea..09572b5 100644
--- a/misc/TODO.txt
+++ b/misc/TODO.txt
@@ -23,3 +23,4 @@
 [ ] When WAL entries are partially written, remove partial data and fail
 [ ] Find a way to ensure listing operations of large directories are handled
 [ ] What happens if a client adds a chunk such that the hash already exists in the system? The client doesn't know it exists already. Will this lead to over-replication?
+[ ] Rename generation counters to version numbers
diff --git a/src/client.c b/src/client.c
index e8d3d9e..7629873 100644
--- a/src/client.c
+++ b/src/client.c
@@ -123,6 +123,7 @@ typedef struct {
     int      num_hashes;
     uint32_t num_chunks;
     uint32_t chunk_size;
+    uint64_t expect_gen;
     UploadSchedule *uploads;
     int num_uploads;
     int cap_uploads;
@@ -598,7 +599,8 @@ ToastyHandle toasty_begin_create_file(ToastyFS *toasty, ToastyString path,
     return begin_create(toasty, path, false, chunk_size);
 }
 
-ToastyHandle toasty_begin_delete(ToastyFS *toasty, ToastyString path)
+ToastyHandle toasty_begin_delete(ToastyFS *toasty,
+    ToastyString path, ToastyVersionTag vtag)
 {
     int opidx = -1;
     ToastyHandle handle = TOASTY_INVALID;
@@ -618,7 +620,7 @@ ToastyHandle toasty_begin_delete(ToastyFS *toasty, ToastyString path)
         goto unlock_and_exit;
     }
     uint16_t path_len = path.len;
-    uint64_t expect_gen = 0;  // 0 means skip generation check
+    uint64_t expect_gen = vtag;
 
     MessageWriter writer;
     metadata_server_request_start(toasty, &writer, MESSAGE_TYPE_DELETE);
@@ -642,7 +644,8 @@ unlock_and_exit:
     return handle;
 }
 
-ToastyHandle toasty_begin_list(ToastyFS *toasty, ToastyString path)
+ToastyHandle toasty_begin_list(ToastyFS *toasty,
+    ToastyString path, ToastyVersionTag vtag)
 {
     int opidx = -1;
     ToastyHandle handle = TOASTY_INVALID;
@@ -662,7 +665,7 @@ ToastyHandle toasty_begin_list(ToastyFS *toasty, ToastyString path)
         goto unlock_and_exit;
     }
     uint16_t path_len = path.len;
-    uint64_t expect_gen = 0;  // 0 means skip generation check
+    uint64_t expect_gen = vtag;
 
     MessageWriter writer;
     metadata_server_request_start(toasty, &writer, MESSAGE_TYPE_LIST);
@@ -686,12 +689,14 @@ unlock_and_exit:
     return handle;
 }
 
-static int send_read_message(ToastyFS *toasty, int opidx, int tag, ToastyString path, uint32_t offset, uint32_t length)
+static int send_read_message(ToastyFS *toasty, int opidx,
+    int tag, ToastyString path, uint32_t offset,
+    uint32_t length, ToastyVersionTag vtag)
 {
     if (path.len > UINT16_MAX)
         return -1;
     uint16_t path_len = path.len;
-    uint64_t expect_gen = 0;  // 0 means skip generation check
+    uint64_t expect_gen = vtag;
 
     MessageWriter writer;
     metadata_server_request_start(toasty, &writer, MESSAGE_TYPE_READ);
@@ -705,7 +710,8 @@ static int send_read_message(ToastyFS *toasty, int opidx, int tag, ToastyString
     return 0;
 }
 
-ToastyHandle toasty_begin_read(ToastyFS *toasty, ToastyString path, int off, void *dst, int len)
+ToastyHandle toasty_begin_read(ToastyFS *toasty, ToastyString path,
+    int off, void *dst, int len, ToastyVersionTag vtag)
 {
     int opidx = -1;
     ToastyHandle handle = TOASTY_INVALID;
@@ -719,7 +725,7 @@ ToastyHandle toasty_begin_read(ToastyFS *toasty, ToastyString path, int off, voi
     if (opidx < 0)
         goto unlock_and_exit;
 
-    if (send_read_message(toasty, opidx, TAG_RETRIEVE_METADATA_FOR_READ, path, off, len) < 0) {
+    if (send_read_message(toasty, opidx, TAG_RETRIEVE_METADATA_FOR_READ, path, off, len, vtag) < 0) {
         free_operation(toasty, opidx);
         opidx = -1;
         goto unlock_and_exit;
@@ -736,7 +742,9 @@ unlock_and_exit:
     return handle;
 }
 
-ToastyHandle toasty_begin_write(ToastyFS *toasty, ToastyString path, int off, void *src, int len)
+ToastyHandle toasty_begin_write(ToastyFS *toasty,
+    ToastyString path, int off, void *src, int len,
+    ToastyVersionTag vtag)
 {
     int opidx = -1;
     ToastyHandle handle = TOASTY_INVALID;
@@ -752,7 +760,7 @@ ToastyHandle toasty_begin_write(ToastyFS *toasty, ToastyString path, int off, vo
 
     toasty->operations[opidx].path = path; // TODO: must be a copy
 
-    if (send_read_message(toasty, opidx, TAG_RETRIEVE_METADATA_FOR_WRITE, path, off, len) < 0) {
+    if (send_read_message(toasty, opidx, TAG_RETRIEVE_METADATA_FOR_WRITE, path, off, len, vtag) < 0) {
         free_operation(toasty, opidx);
         opidx = -1;
         goto unlock_and_exit;
@@ -1462,6 +1470,7 @@ static void process_event_for_write(ToastyFS *toasty,
             toasty->operations[opidx].result = (ToastyResult) { .type=TOASTY_RESULT_WRITE_ERROR, .user=toasty->operations[opidx].user };
             return;
         }
+        toasty->operations[opidx].expect_gen = gen;
 
         uint32_t chunk_size;
         if (!binary_read(&reader, &chunk_size, sizeof(chunk_size))) {
@@ -1961,7 +1970,7 @@ static void process_event_for_write(ToastyFS *toasty,
                 assert(0); // TODO
             }
             uint16_t path_len = path.len;
-            uint64_t expect_gen = 0;  // 0 means skip generation check - but per protocol, WRITE can't use 0
+            uint64_t expect_gen = toasty->operations[opidx].expect_gen;
 
             uint32_t num_chunks = num_upload_results;
 
@@ -2315,7 +2324,8 @@ int toasty_wait_result(ToastyFS *toasty, ToastyHandle handle, ToastyResult *resu
     return 0;
 }
 
-int toasty_create_dir(ToastyFS *toasty, ToastyString path)
+int toasty_create_dir(ToastyFS *toasty, ToastyString path,
+    ToastyVersionTag *vtag)
 {
     ToastyHandle handle = toasty_begin_create_dir(toasty, path);
     if (handle == TOASTY_INVALID)
@@ -2330,12 +2340,13 @@ int toasty_create_dir(ToastyFS *toasty, ToastyString path)
         return -1;
     }
 
+    if (vtag) *vtag = result.vtag;
     toasty_free_result(&result);
     return 0;
 }
 
 int toasty_create_file(ToastyFS *toasty, ToastyString path,
-    unsigned int chunk_size)
+    unsigned int chunk_size, ToastyVersionTag *vtag)
 {
     ToastyHandle handle = toasty_begin_create_file(toasty, path, chunk_size);
     if (handle == TOASTY_INVALID)
@@ -2350,13 +2361,15 @@ int toasty_create_file(ToastyFS *toasty, ToastyString path,
         return -1;
     }
 
+    if (vtag) *vtag = result.vtag;
     toasty_free_result(&result);
     return 0;
 }
 
-int toasty_delete(ToastyFS *toasty, ToastyString path)
+int toasty_delete(ToastyFS *toasty, ToastyString path,
+    ToastyVersionTag vtag)
 {
-    ToastyHandle handle = toasty_begin_delete(toasty, path);
+    ToastyHandle handle = toasty_begin_delete(toasty, path, vtag);
     if (handle == TOASTY_INVALID)
         return -1;
 
@@ -2373,9 +2386,10 @@ int toasty_delete(ToastyFS *toasty, ToastyString path)
     return 0;
 }
 
-int toasty_list(ToastyFS *toasty, ToastyString path, ToastyListing *listing)
+int toasty_list(ToastyFS *toasty, ToastyString path,
+    ToastyListing *listing, ToastyVersionTag *vtag)
 {
-    ToastyHandle handle = toasty_begin_list(toasty, path);
+    ToastyHandle handle = toasty_begin_list(toasty, path, vtag ? *vtag : 0);
     if (handle == TOASTY_INVALID)
         return -1;
 
@@ -2388,6 +2402,9 @@ int toasty_list(ToastyFS *toasty, ToastyString path, ToastyListing *listing)
         return -1;
     }
 
+    if (vtag)
+        *vtag = result.vtag;
+
     if (listing)
         *listing = result.listing;
     else
@@ -2401,9 +2418,9 @@ void toasty_free_listing(ToastyListing *listing)
 }
 
 int toasty_read(ToastyFS *toasty, ToastyString path,
-    int off, void *dst, int len)
+    int off, void *dst, int len, ToastyVersionTag *vtag)
 {
-    ToastyHandle handle = toasty_begin_read(toasty, path, off, dst, len);
+    ToastyHandle handle = toasty_begin_read(toasty, path, off, dst, len, vtag ? *vtag : 0);
     if (handle == TOASTY_INVALID)
         return -1;
 
@@ -2416,15 +2433,16 @@ int toasty_read(ToastyFS *toasty, ToastyString path,
         return -1;
     }
 
+    if (vtag) *vtag = result.vtag;
     int bytes_read = result.bytes_read;
     toasty_free_result(&result);
     return bytes_read;
 }
 
 int toasty_write(ToastyFS *toasty, ToastyString path,
-    int off, void *src, int len)
+    int off, void *src, int len, ToastyVersionTag *vtag)
 {
-    ToastyHandle handle = toasty_begin_write(toasty, path, off, src, len);
+    ToastyHandle handle = toasty_begin_write(toasty, path, off, src, len, vtag ? *vtag : 0);
     if (handle == TOASTY_INVALID)
         return -1;
 
@@ -2437,6 +2455,7 @@ int toasty_write(ToastyFS *toasty, ToastyString path,
         return -1;
     }
 
+    if (vtag) *vtag = result.vtag;
     toasty_free_result(&result);
     return 0; // TODO: return the number of bytes written?
 }
diff --git a/src/simulation_client.c b/src/simulation_client.c
index eaf8a80..6cbe595 100644
--- a/src/simulation_client.c
+++ b/src/simulation_client.c
@@ -182,13 +182,13 @@ int simulation_client_step(SimulationClient *client, void **contexts,
 
             case PENDING_OPERATION_DELETE:
             entry = table[random_in_range(0, table_len-1)];
-            handle = toasty_begin_delete(client->toasty, entry.path);
+            handle = toasty_begin_delete(client->toasty, entry.path, TOASTY_VERSION_TAG_EMPTY);
             //printf("[Client] submit delete (path=%s)\n", entry.path);
             break;
 
             case PENDING_OPERATION_LIST:
             entry = table[random_in_range(0, table_len-1)];
-            handle = toasty_begin_list(client->toasty, entry.path);
+            handle = toasty_begin_list(client->toasty, entry.path, TOASTY_VERSION_TAG_EMPTY);
             //printf("[Client] submit list   (path=%s)\n", entry.path);
             break;
 
@@ -198,7 +198,7 @@ int simulation_client_step(SimulationClient *client, void **contexts,
             len = random_in_range(0, 5000);
             ptr = malloc(len);
             if (ptr == NULL) assert(0);
-            handle = toasty_begin_read(client->toasty, entry.path, off, ptr, len);
+            handle = toasty_begin_read(client->toasty, entry.path, off, ptr, len, TOASTY_VERSION_TAG_EMPTY);
             //printf("[Client] submit read   (path=%s, off=%d, len=%d)\n", entry.path, off, len);
             break;
 
@@ -209,7 +209,7 @@ int simulation_client_step(SimulationClient *client, void **contexts,
             ptr = malloc(len);
             if (ptr == NULL) assert(0);
             memset(ptr, 'a', len);
-            handle = toasty_begin_write(client->toasty, entry.path, off, ptr, len);
+            handle = toasty_begin_write(client->toasty, entry.path, off, ptr, len, TOASTY_VERSION_TAG_EMPTY);
             //printf("[Client] submit write  (path=%s, off=%d, len=%d)\n", entry.path, off, len);
             break;
         }
diff --git a/web/main.c b/web/main.c
index e82964f..bf48db9 100644
--- a/web/main.c
+++ b/web/main.c
@@ -292,7 +292,7 @@ int main(int argc, char **argv)
                                 proxied[i].request->url.path.ptr,
                                 proxied[i].request->url.path.len,
                             };
-                            proxied[i].handle = toasty_begin_read(toasty, path, proxied[i].transferred, dst, cap);
+                            proxied[i].handle = toasty_begin_read(toasty, path, proxied[i].transferred, dst, cap, TOASTY_VERSION_TAG_EMPTY);
                             if (proxied[i].handle == TOASTY_INVALID) {
                                 assert(0); // TODO
                             }
@@ -355,11 +355,22 @@ int main(int argc, char **argv)
                         break;
                     }
 
+                    ToastyVersionTag vtag;
+                    {
+                        int i = http_find_header(request->headers, request->num_headers, HTTP_STR("ETag"));
+                        if (i < 0)
+                            vtag = TOASTY_VERSION_TAG_EMPTY;
+                        else {
+                            // TODO: parse ETag and store its value into vtag
+                            vtag = -1;
+                        }
+                    }
+
                     ToastyString path = {
                         request->url.path.ptr,
                         request->url.path.len,
                     };
-                    ToastyHandle handle = toasty_begin_write(toasty, path, 0, request->body.ptr, request->body.len);
+                    ToastyHandle handle = toasty_begin_write(toasty, path, 0, request->body.ptr, request->body.len, vtag);
                     if (handle == TOASTY_INVALID) {
                         http_response_builder_status(builder, 500); // Internal Server Error
                         http_response_builder_send(builder);
@@ -383,11 +394,22 @@ int main(int argc, char **argv)
                         break;
                     }
 
+                    ToastyVersionTag vtag;
+                    {
+                        int i = http_find_header(request->headers, request->num_headers, HTTP_STR("ETag"));
+                        if (i < 0)
+                            vtag = TOASTY_VERSION_TAG_EMPTY;
+                        else {
+                            // TODO: parse ETag and store its value into vtag
+                            vtag = -1;
+                        }
+                    }
+
                     ToastyString path = {
                         request->url.path.ptr,
                         request->url.path.len,
                     };
-                    ToastyHandle handle = toasty_begin_delete(toasty, path);
+                    ToastyHandle handle = toasty_begin_delete(toasty, path, vtag);
                     if (handle == TOASTY_INVALID) {
                         http_response_builder_status(builder, 500); // Internal Server Error
                         http_response_builder_send(builder);