diff --git a/mercurial/help.py b/mercurial/help.py
--- a/mercurial/help.py
+++ b/mercurial/help.py
@@ -103,6 +103,7 @@ helptable = [
      loaddoc('templates')),
     (['urls'], _('URL Paths'), loaddoc('urls')),
     (["extensions"], _("Using additional features"), extshelp),
+    (["subrepo", "subrepos"], _("Subrepositories"), loaddoc('subrepos')),
     (["hgweb"], _("Configuring hgweb"), loaddoc('hgweb')),
     (["glossary"], _("Glossary"), loaddoc('glossary')),
 ]
diff --git a/mercurial/help/subrepos.txt b/mercurial/help/subrepos.txt
new file mode 100644
--- /dev/null
+++ b/mercurial/help/subrepos.txt
@@ -0,0 +1,128 @@
+Subrepositories let you nest external repositories or projects into a
+parent Mercurial repository, and make commands operate on them as a
+group. External Mercurial and Subversion projects are currently
+supported.
+
+Subrepositories are made of three components:
+
+1. Nested repository checkouts. They can appear anywhere in the
+   parent working directory, and are Mercurial clones or Subversion
+   checkouts.
+
+2. Nested repository references. They are defined in ``.hgsub`` and
+   tell where the subrepository checkouts come from. Mercurial
+   subrepositories are referenced like:
+
+     path/to/nested = https://example.com/nested/repo/path
+
+   where ``path/to/nested`` is the checkout location relatively to the
+   parent Mercurial root, and ``https://example.com/nested/repo/path``
+   is the source repository path. The source can also reference a
+   filesystem path. Subversion repositories are defined with:
+
+     path/to/nested = [svn]https://example.com/nested/trunk/path
+
+   Note that ``.hgsub`` does not exist by default in Mercurial
+   repositories, you have to create and add it to the parent
+   repository before using subrepositories.
+
+3. Nested repository states. They are defined in ``.hgsubstate`` and
+   capture whatever information is required to restore the
+   subrepositories to the state they were committed in a parent
+   repository changeset. Mercurial automatically record the nested
+   repositories states when committing in the parent repository.
+
+   .. note::
+      The ``.hgsubstate`` file should not be edited manually.
+
+
+Adding a Subrepository
+----------------------
+
+If ``.hgsub`` does not exist, create it and add it to the parent
+repository. Clone or checkout the external projects where you want it
+to live in the parent repository. Edit ``.hgsub`` and add the
+subrepository entry as described above. At this point, the
+subrepository is tracked and the next commit will record its state in
+``.hgsubstate`` and bind it to the committed changeset.
+
+Synchronizing a Subrepository
+-----------------------------
+
+Subrepos do not automatically track the latest changeset of their
+sources. Instead, they are updated to the changeset that corresponds
+with the changeset checked out in the top-level changeset. This is so
+developers always get a consistent set of compatible code and
+libraries when they update.
+
+Thus, updating subrepos is a manual process. Simply check out target
+subrepo at the desired revision, test in the top-level repo, then
+commit in the parent repository to record the new combination.
+
+Deleting a Subrepository
+------------------------
+
+To remove a subrepo from the parent repository, delete its reference
+from ``.hgsub``. Then, the subrepo tree will show up as a set of
+unknown files in :hg:`status`, and you can delete the files.
+
+Interaction with Mercurial Commands
+-----------------------------------
+
+:add: add does not recurse in subrepos unless -S/--subrepos is
+    specified. Subversion subrepositories are currently silently
+    ignored.
+
+:archive: archive does not recurse in subrepositories unless
+    -S/--subrepos is specified.
+
+:commit: commit creates a consistent snapshot of the state of the
+    entire project and its subrepositories. It does this by first
+    attempting to commit all modified subrepositories, then recording
+    their state and finally committing it in the parent repository.
+
+:diff: diff does not recurse in subrepos unless -S/--subrepos is
+    specified. Changes are displayed as usual, on the subrepositories
+    elements. Subversion subrepositories are currently silently
+    ignored.
+
+:incoming: incoming does not recurse in subrepos unless -S/--subrepos
+    is specified. Subversion subrepositories are currently silently
+    ignored.
+
+:outgoing: outgoing does not recurse in subrepos unless -S/--subrepos
+    is specified. Subversion subrepositories are currently silently
+    ignored.
+
+:pull: pull is not recursive since it is not clear what to pull prior
+    to running :hg:`update`. Listing and retrieving all
+    subrepositories changes referenced by the parent repository pulled
+    changesets is expensive at best, impossible in the Subversion
+    case.
+
+:push: Mercurial will automatically push all subrepositories first
+    when the parent repository is being pushed. This ensures new
+    subrepository changes are available when referenced by top-level
+    repositories.
+
+:status: status does not recurse into subrepositories unless
+    -S/--subrepos is specified. Subrepository changes are displayed as
+    regular Mercurial changes on the subrepository
+    elements. Subversion subrepositories are currently silently
+    ignored.
+
+:update: update restores the subrepos in the state they were
+    originally committed in target changeset. If the recorded
+    changeset is not available in the current subrepository, Mercurial
+    will pull it in first before updating.  This means that updating
+    can require network access when using subrepositories.
+
+Remapping Subrepositories Sources
+---------------------------------
+
+A subrepository source location may change during a project life,
+invalidating references stored in the parent repository history. To
+fix this, rewriting rules can be defined in parent repository ``hgrc``
+file or in Mercurial configuration. See the ``[subpaths]`` section in
+hgrc(5) for more details.
+
diff --git a/tests/test-globalopts.t b/tests/test-globalopts.t
--- a/tests/test-globalopts.t
+++ b/tests/test-globalopts.t
@@ -343,6 +343,7 @@ Testing -h/--help:
    templating   Template Usage
    urls         URL Paths
    extensions   Using additional features
+   subrepos     Subrepositories
    hgweb        Configuring hgweb
    glossary     Glossary
   
@@ -418,6 +419,7 @@ Testing -h/--help:
    templating   Template Usage
    urls         URL Paths
    extensions   Using additional features
+   subrepos     Subrepositories
    hgweb        Configuring hgweb
    glossary     Glossary
   
diff --git a/tests/test-help.t b/tests/test-help.t
--- a/tests/test-help.t
+++ b/tests/test-help.t
@@ -114,6 +114,7 @@ Short help:
    templating   Template Usage
    urls         URL Paths
    extensions   Using additional features
+   subrepos     Subrepositories
    hgweb        Configuring hgweb
    glossary     Glossary
   
@@ -185,6 +186,7 @@ Short help:
    templating   Template Usage
    urls         URL Paths
    extensions   Using additional features
+   subrepos     Subrepositories
    hgweb        Configuring hgweb
    glossary     Glossary
 
@@ -708,6 +710,7 @@ Test that default list of commands omits
    templating   Template Usage
    urls         URL Paths
    extensions   Using additional features
+   subrepos     Subrepositories
    hgweb        Configuring hgweb
    glossary     Glossary