diff --git a/qemu-img-cmds.hx b/qemu-img-cmds.hx
index 49dce7c..01a9246 100644
--- a/qemu-img-cmds.hx
+++ b/qemu-img-cmds.hx
@@ -33,6 +33,12 @@ STEXI
 @item convert [-c] [-p] [-f @var{fmt}] [-t @var{cache}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] [-S @var{sparse_size}] @var{filename} [@var{filename2} [...]] @var{output_filename}
 ETEXI
 
+DEF("diff", img_diff,
+    "diff [-f fmt] [-F backing_fmt] [-O output_fmt] -b backing_file filename output_filename")
+STEXI
+@item rebase [-f @var{fmt}] [-F @var{backing_fmt}] [-O @var{output_fmt}] -b @var{backing_file} @var{filename} @var{output_filename}
+ETEXI
+
 DEF("info", img_info,
     "info [-f fmt] filename")
 STEXI
diff --git a/qemu-img.c b/qemu-img.c
index c8a70ff..6e3fe2a 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1533,6 +1533,156 @@ out:
     return 0;
 }
 
+static int img_diff(int argc, char **argv)
+{
+    /* qemu-img diff -b original modified out */
+    BlockDriverState *bs_original, *bs_modified, *bs_out;
+    const char *fmt_original, *original,
+        *fmt_modified, *modified,
+        *fmt_out, *out;
+    int c, ret = 0;
+    uint64_t num_sectors, modified_num_sectors;
+    uint64_t sector;
+    int n;
+    uint8_t *buf_original;
+    uint8_t *buf_modified;
+
+    /* Parse commandline parameters */
+    fmt_original = NULL;
+    fmt_modified = NULL;
+    fmt_out = NULL;
+    original = NULL;
+    for(;;) {
+        c = getopt(argc, argv, "hf:F:b:O:");
+        if (c == -1) {
+            break;
+        }
+        switch(c) {
+        case '?':
+        case 'h':
+            help();
+            return 0;
+        case 'f':
+            fmt_modified = optarg;
+            break;
+        case 'F':
+            fmt_original = optarg;
+            break;
+        case 'b':
+            original = optarg;
+            break;
+        case 'O':
+            fmt_out = optarg;
+            break;
+        }
+    }
+
+    if (original == NULL) {
+        error_report("The -b (backing filename) option must be supplied");
+        return 1;
+    }
+
+    if (argc - optind != 2) {
+        error_report("The input and output filenames must be supplied");
+        return 1;
+    }
+    modified = argv[optind++];
+    out = argv[optind++];
+
+    /* Open the input images. */
+    bs_original = bdrv_new_open(original, fmt_original, BDRV_O_FLAGS);
+    if (!bs_original) {
+        return 1;
+    }
+
+    bs_modified = bdrv_new_open(modified, fmt_modified, BDRV_O_FLAGS);
+    if (!bs_modified) {
+        return 1;
+    }
+
+    bdrv_get_geometry(bs_original, &num_sectors);
+    bdrv_get_geometry(bs_modified, &modified_num_sectors);
+    if (num_sectors != modified_num_sectors) {
+        error_report("Number of sectors in backing and source must be the same");
+        goto out2;
+    }
+
+    /* Output image. */
+    if (fmt_out == NULL || fmt_out[0] == '\0') {
+        fmt_out = "qcow2";
+    }
+    ret = bdrv_img_create(out, fmt_out,
+                          /* original file becomes the new backing file */
+                          original, fmt_original,
+                          NULL, num_sectors * BDRV_SECTOR_SIZE, BDRV_O_FLAGS);
+    if (ret != 0) {
+        goto out2;
+    }
+    bs_out = bdrv_new_open(out, fmt_out, BDRV_O_RDWR);
+
+    buf_original = qemu_blockalign(bs_original, IO_BUF_SIZE);
+    buf_modified = qemu_blockalign(bs_modified, IO_BUF_SIZE);
+
+    for (sector = 0; sector < num_sectors; sector += n) {
+        /* How many sectors can we handle with the next read? */
+        if (sector + (IO_BUF_SIZE / BDRV_SECTOR_SIZE) <= num_sectors) {
+            n = IO_BUF_SIZE / BDRV_SECTOR_SIZE;
+        } else {
+            n = num_sectors - sector;
+        }
+
+        /* Read input files and compare. */
+        ret = bdrv_read(bs_original, sector, buf_original, n);
+        if (ret < 0) {
+            error_report("error while reading from backing file");
+            goto out;
+        }
+
+        ret = bdrv_read(bs_modified, sector, buf_modified, n);
+        if (ret < 0) {
+            error_report("error while reading from input file");
+            goto out;
+        }
+
+        /* If they differ, we need to write to the differences file. */
+        uint64_t written = 0;
+
+        while (written < n) {
+            int pnum;
+
+            if (compare_sectors(buf_original + written * BDRV_SECTOR_SIZE,
+                                buf_modified + written * BDRV_SECTOR_SIZE,
+                                n - written, &pnum)) {
+                ret = bdrv_write(bs_out, sector + written,
+                                 buf_modified + written * BDRV_SECTOR_SIZE,
+                                 pnum);
+                if (ret < 0) {
+                    error_report("Error while writing to output file: %s",
+                                 strerror(-ret));
+                    goto out;
+                }
+            }
+
+            written += pnum;
+        }
+    }
+
+    qemu_vfree(buf_original);
+    qemu_vfree(buf_modified);
+
+ out:
+    /* Cleanup */
+    bdrv_delete(bs_out);
+ out2:
+    bdrv_delete(bs_original);
+    bdrv_delete(bs_modified);
+
+    if (ret) {
+        return 1;
+    }
+    return 0;
+}
+
 static int img_resize(int argc, char **argv)
 {
     int c, ret, relative;
diff --git a/qemu-img.texi b/qemu-img.texi
index b2ca3a5..e1a123b 100644
--- a/qemu-img.texi
+++ b/qemu-img.texi
@@ -114,6 +114,23 @@ created as a copy on write image of the specified base image; the
 @var{backing_file} should have the same content as the input's base image,
 however the path, image format, etc may differ.
 
+@item diff [-f @var{fmt}] [-F @var{backing_fmt}] [-O @var{output_fmt}] -b @var{backing_file} @var{filename} @var{output_filename}
+
+Create a new file (@var{output_filename}) which contains the
+differences between @var{backing_file} and @var{filename}.
+
+The @var{backing_file} and @var{filename} must have the same
+virtual disk size, but may be in different formats.
+
+@var{output_file} will have @var{backing_file} set as its backing
+file.  The format of @var{output_file} must be one that supports
+backing files (currently @code{qcow2} is the default and only
+permitted output format).
+
+Typical usage is:
+
+@code{qemu-img diff -b original.img modified.img diff.qcow2}
+
 @item info [-f @var{fmt}] @var{filename}
 
 Give information about the disk image @var{filename}. Use it in
