diff --git a/src/testdir/test_undo.vim b/src/testdir/test_undo.vim
new file mode 100644
index 0000000..e521e35
--- /dev/null
+++ b/src/testdir/test_undo.vim
@@ -0,0 +1,204 @@
+" Tests for the undo tree.
+" Since this script is sourced we need to explicitly break changes up in
+" undo-able pieces.  Do that by setting 'undolevels'.
+" Also tests :earlier and :later.
+
+func Test_undotree()
+  exe "normal Aabc\<Esc>"
+  set ul=100
+  exe "normal Adef\<Esc>"
+  set ul=100
+  undo
+  let d = undotree()
+  call assert_true(d.seq_last > 0)
+  call assert_true(d.seq_cur > 0)
+  call assert_true(d.seq_cur < d.seq_last)
+  call assert_true(len(d.entries) > 0)
+  " TODO: check more members of d
+
+  w! Xtest
+  call assert_equal(d.save_last + 1, undotree().save_last)
+  call delete('Xtest')
+  bwipe Xtest
+endfunc
+
+func FillBuffer()
+  for i in range(1,13)
+    put=i
+    " Set 'undolevels' to split undo. 
+    exe "setg ul=" . &g:ul
+  endfor
+endfunc
+
+func Test_global_local_undolevels()
+  new one
+  set undolevels=5
+  call FillBuffer()
+  " will only undo the last 5 changes, end up with 13 - (5 + 1) = 7 lines
+  earlier 10
+  call assert_equal(5, &g:undolevels)
+  call assert_equal(-123456, &l:undolevels)
+  call assert_equal('7', getline('$'))
+
+  new two
+  setlocal undolevels=2
+  call FillBuffer()
+  " will only undo the last 2 changes, end up with 13 - (2 + 1) = 10 lines
+  earlier 10
+  call assert_equal(5, &g:undolevels)
+  call assert_equal(2, &l:undolevels)
+  call assert_equal('10', getline('$'))
+
+  setlocal ul=10
+  call assert_equal(5, &g:undolevels)
+  call assert_equal(10, &l:undolevels)
+
+  " Setting local value in "two" must not change local value in "one"
+  wincmd p
+  call assert_equal(5, &g:undolevels)
+  call assert_equal(-123456, &l:undolevels)
+
+  new three
+  setglobal ul=50
+  call assert_equal(50, &g:undolevels)
+  call assert_equal(-123456, &l:undolevels)
+
+  " Drop created windows
+  set ul&
+  new
+  only!
+endfunc
+
+func BackOne(expected)
+  call feedkeys('g-', 'xt')
+  call assert_equal(a:expected, getline(1))
+endfunc
+
+func Test_undo_del_chars()
+  " Setup a buffer without creating undo entries
+  new
+  set ul=-1
+  call setline(1, ['123-456'])
+  set ul=100
+  1
+  call test_settime(100)
+
+  " Delete three characters and undo with g-
+  call feedkeys('x', 'xt')
+  call feedkeys('x', 'xt')
+  call feedkeys('x', 'xt')
+  call assert_equal('-456', getline(1))
+  call BackOne('3-456')
+  call BackOne('23-456')
+  call BackOne('123-456')
+  call assert_fails("BackOne('123-456')")
+
+  :" Delete three other characters and go back in time with g-
+  call feedkeys('$x', 'xt')
+  call feedkeys('x', 'xt')
+  call feedkeys('x', 'xt')
+  call assert_equal('123-', getline(1))
+  call test_settime(101)
+
+  call BackOne('123-4')
+  call BackOne('123-45')
+  " skips '123-456' because it's older
+  call BackOne('-456')
+  call BackOne('3-456')
+  call BackOne('23-456')
+  call BackOne('123-456')
+  call assert_fails("BackOne('123-456')")
+  normal 10g+
+  call assert_equal('123-', getline(1))
+
+  :" Jump two seconds and go some seconds forward and backward
+  call test_settime(103)
+  call feedkeys("Aa\<Esc>", 'xt')
+  call feedkeys("Ab\<Esc>", 'xt')
+  call feedkeys("Ac\<Esc>", 'xt')
+  call assert_equal('123-abc', getline(1))
+  earlier 1s
+  call assert_equal('123-', getline(1))
+  earlier 3s
+  call assert_equal('123-456', getline(1))
+  later 1s
+  call assert_equal('123-', getline(1))
+  later 1h
+  call assert_equal('123-abc', getline(1))
+
+  close!
+endfunc
+
+func Test_undojoin()
+  new
+  call feedkeys("Goaaaa\<Esc>", 'xt')
+  call feedkeys("obbbb\<Esc>", 'xt')
+  call assert_equal(['aaaa', 'bbbb'], getline(2, '$'))
+  call feedkeys("u", 'xt')
+  call assert_equal(['aaaa'], getline(2, '$'))
+  call feedkeys("obbbb\<Esc>", 'xt')
+  undojoin
+  " Note: next change must not be as if typed
+  call feedkeys("occcc\<Esc>", 'x')
+  call assert_equal(['aaaa', 'bbbb', 'cccc'], getline(2, '$'))
+  call feedkeys("u", 'xt')
+  call assert_equal(['aaaa'], getline(2, '$'))
+  close!
+endfunc
+
+func Test_undo_write()
+  split Xtest
+  call feedkeys("ione one one\<Esc>", 'xt')
+  w!
+  call feedkeys("otwo\<Esc>", 'xt')
+  call feedkeys("otwo\<Esc>", 'xt')
+  w
+  call feedkeys("othree\<Esc>", 'xt')
+  call assert_equal(['one one one', 'two', 'two', 'three'], getline(1, '$'))
+  earlier 1f
+  call assert_equal(['one one one', 'two', 'two'], getline(1, '$'))
+  earlier 1f
+  call assert_equal(['one one one'], getline(1, '$'))
+  earlier 1f
+  call assert_equal([''], getline(1, '$'))
+  later 1f
+  call assert_equal(['one one one'], getline(1, '$'))
+  later 1f
+  call assert_equal(['one one one', 'two', 'two'], getline(1, '$'))
+  later 1f
+  call assert_equal(['one one one', 'two', 'two', 'three'], getline(1, '$'))
+
+  close!
+  call delete('Xtest')
+  bwipe! Xtest
+endfunc
+
+func Test_insert_expr()
+  new
+  " calling setline() triggers undo sync
+  call feedkeys("oa\<Esc>", 'xt')
+  call feedkeys("ob\<Esc>", 'xt')
+  set ul=100
+  call feedkeys("o1\<Esc>a2\<C-R>=setline('.','1234')\<CR>\<CR>\<Esc>", 'x')
+  call assert_equal(['a', 'b', '120', '34'], getline(2, '$'))
+  call feedkeys("u", 'x')
+  call assert_equal(['a', 'b', '12'], getline(2, '$'))
+  call feedkeys("u", 'x')
+  call assert_equal(['a', 'b'], getline(2, '$'))
+
+  call feedkeys("oc\<Esc>", 'xt')
+  set ul=100
+  call feedkeys("o1\<Esc>a2\<C-R>=setline('.','1234')\<CR>\<CR>\<Esc>", 'x')
+  call assert_equal(['a', 'b', 'c', '120', '34'], getline(2, '$'))
+  call feedkeys("u", 'x')
+  call assert_equal(['a', 'b', 'c', '12'], getline(2, '$'))
+
+  call feedkeys("od\<Esc>", 'xt')
+  set ul=100
+  call feedkeys("o1\<Esc>a2\<C-R>=string(123)\<CR>\<Esc>", 'x')
+  call assert_equal(['a', 'b', 'c', '12', 'd', '12123'], getline(2, '$'))
+  call feedkeys("u", 'x')
+  call assert_equal(['a', 'b', 'c', '12', 'd'], getline(2, '$'))
+
+  close!
+endfunc