diff --git a/git-imerge b/git-imerge
index 457fdab..9caa9e9 100755
--- a/git-imerge
+++ b/git-imerge
@@ -545,7 +545,7 @@ def reparent(commit, parent_sha1s, msg=None):
     return out.strip()
 
 
-class AutomaticMergeFailed(Exception):
+class LogicalFailure(Exception):
     def __init__(self, commit1, commit2):
         Exception.__init__(
             self, 'Automatic merge of %s and %s failed' % (commit1, commit2,)
@@ -553,7 +553,76 @@ class AutomaticMergeFailed(Exception):
         self.commit1, self.commit2 = commit1, commit2
 
 
-def automerge(commit1, commit2, msg=None):
+class AutomaticMergeFailed(LogicalFailure):
+    pass
+
+
+class AutomaticTestFailed(LogicalFailure):
+    pass
+
+
+class GitConfigError(Exception):
+    def __init__(self, returncode, output):
+        Exception.__init__(
+            self, 'Git config failed with exit code %s: %s' % (returncode, output,)
+            )
+
+
+def memo(obj):
+    cache = {}
+    @functools.wraps(obj)
+    def wrap(*args, **kwds):
+        if args not in cache:
+            cache[args] = obj(*args, **kwds)
+        return cache[args]
+    return wrap
+
+
+@memo
+class GitConfigStore(object):
+    def __init__(self, name, config_prefix='imerge'):
+        self.config_prefix = config_prefix
+        self.config = self._get_all_keys()
+
+    def _get_all_keys(self):
+        d = {}
+        try:
+            items_with_prefix = check_output(
+                ['git', 'config', '--get-regex', self.config_prefix]
+                ).rstrip().split('\n')
+            for row in items_with_prefix:
+                k, v = row.split()
+                d[k[len(self.config_prefix + '.'):]] = v
+            return d
+        except CalledProcessError:
+            return {}
+
+    def get(self, key):
+        return self.config.get(key)
+
+    def set(self, key, value):
+        self.config[key] = value
+        config_key = '.'.join([self.config_prefix, key])
+        try:
+            check_call(['git', 'config', config_key, value])
+        except CalledProcessError as e:
+            raise GitConfigError(e.returncode, e.output)
+
+    def unset(self, key):
+        if key in self.config:
+            del self.config[key]
+        config_key = '.'.join([self.config_prefix, key])
+        try:
+            check_call(['git', 'config', '--unset', config_key])
+        except CalledProcessError as e:
+            if e.returncode == 5:
+                # Value was not set
+                pass
+            else:
+                raise GitConfigError(e.returncode, e.output)
+
+
+def automerge(commit1, commit2, msg=None, test_command=None):
     """Attempt an automatic merge of commit1 and commit2.
 
     Return the SHA1 of the resulting commit, or raise
@@ -572,8 +641,14 @@ def automerge(commit1, commit2, msg=None):
         # added in git version 1.7.4.
         call_silently(['git', 'reset', '--merge'])
         raise AutomaticMergeFailed(commit1, commit2)
-    else:
-        return get_commit_sha1('HEAD')
+
+    if test_command is not None:
+        try:
+            check_call(['/bin/sh', '-c', test_command])
+        except CalledProcessError as e:
+            raise AutomaticTestFailed(commit1, commit2)
+
+    return get_commit_sha1('HEAD')
 
 
 class MergeRecord(object):
@@ -1374,6 +1449,7 @@ class Block(object):
         self.name = name
         self.len1 = len1
         self.len2 = len2
+        self.gcs  = GitConfigStore(name)
 
     def get_merge_state(self):
         """Return the MergeState instance containing this Block."""
@@ -1474,11 +1550,17 @@ class Block(object):
                 'Attempting automerge of %d-%d...' % self.get_original_indexes(i1, i2)
                 )
             try:
-                automerge(self[i1, 0].sha1, self[0, i2].sha1)
+                print("Automerging from is_mergeable")
+                automerge(self[i1, 0].sha1, self[0, i2].sha1,
+                          test_command=self.gcs.get(self.name + '.testcommand'),
+                          )
                 sys.stderr.write('success.\n')
                 return True
             except AutomaticMergeFailed:
-                sys.stderr.write('failure.\n')
+                sys.stderr.write('merge failure.\n')
+                return False
+            except AutomaticTestFailed:
+                sys.stderr.write('test failure.\n')
                 return False
 
     def auto_outline(self):
@@ -1497,7 +1579,10 @@ class Block(object):
             sys.stderr.write(msg % (i1orig, i2orig))
             logmsg = 'imerge \'%s\': automatic merge %d-%d' % (self.name, i1orig, i2orig)
             try:
-                merge = automerge(commit1, commit2, msg=logmsg)
+                print("Automerging from auto_outline")
+                merge = automerge(commit1, commit2, msg=logmsg,
+                                  test_command=self.gcs.get(self.name + '.testcommand'),
+                                  )
                 sys.stderr.write('success.\n')
             except AutomaticMergeFailed as e:
                 sys.stderr.write('unexpected conflict.  Backtracking...\n')
@@ -1571,10 +1656,12 @@ class Block(object):
         sys.stderr.write('Attempting to merge %d-%d...' % (i1orig, i2orig))
         logmsg = 'imerge \'%s\': automatic merge %d-%d' % (self.name, i1orig, i2orig)
         try:
+            print("Automerging from auto_fill_micromerge")
             merge = automerge(
                 self[i1, i2 - 1].sha1,
                 self[i1 - 1, i2].sha1,
                 msg=logmsg,
+                test_command=self.gcs.get(self.name + '.testcommand'),
                 )
             sys.stderr.write('success.\n')
         except AutomaticMergeFailed:
@@ -1778,6 +1865,8 @@ class MergeState(Block):
         re.VERBOSE,
         )
 
+    DEFAULT_TEST_COMMAND = None
+
     @staticmethod
     def iter_existing_names():
         """Iterate over the names of existing MergeStates in this repo."""
@@ -1838,26 +1927,19 @@ class MergeState(Block):
         """Set the default merge to the specified one.
 
         name can be None to cause the default to be cleared."""
-
+        gcs = GitConfigStore(name)
         if name is None:
-            try:
-                check_call(['git', 'config', '--unset', 'imerge.default'])
-            except CalledProcessError as e:
-                if e.returncode == 5:
-                    # Value was not set
-                    pass
-                else:
-                    raise
+            gcs.unset("default")
         else:
-            check_call(['git', 'config', 'imerge.default', name])
+            gcs.set("default", name)
 
     @staticmethod
     def get_default_name():
         """Get the name of the default merge, or None if none is currently set."""
-
+        gcs = GitConfigStore(None)
         try:
-            return check_output(['git', 'config', 'imerge.default']).rstrip()
-        except CalledProcessError:
+            return gcs.get("default")
+        except GitConfigError:
             return None
 
     @staticmethod
@@ -1891,7 +1973,7 @@ class MergeState(Block):
         name, merge_base,
         tip1, commits1,
         tip2, commits2,
-        goal=DEFAULT_GOAL, manual=False, branch=None,
+        goal=DEFAULT_GOAL, manual=False, branch=None, test_command=None,
         ):
         """Create and return a new MergeState object."""
 
@@ -1915,6 +1997,7 @@ class MergeState(Block):
             goal=goal,
             manual=manual,
             branch=branch,
+            test_command=test_command,
             )
 
     @staticmethod
@@ -2109,6 +2192,7 @@ class MergeState(Block):
         goal=DEFAULT_GOAL,
         manual=False,
         branch=None,
+        test_command=None,
         ):
         Block.__init__(self, name, len(commits1) + 1, len(commits2) + 1)
         self.tip1 = tip1
@@ -2647,6 +2731,20 @@ def read_merge_state(name=None, default_to_unique=True):
 
 @Failure.wrap
 def main(args):
+    def add_test_command_argument(subparser):
+        subparser.add_argument(
+            '--test-command',
+            action='store', default=None,
+            help=(
+                'in addition to identifying for textual conflicts, run the test '
+                'or test script specified by TEST to identify where logical '
+                'conflicts are introduced. The test script is expected to return 0 '
+                'if the source is good, exit with code 1-127 if the source is bad, '
+                'except for exit code 125 which indicates the source code can not '
+                'be built or tested.'
+                ),
+            )
+
     parser = argparse.ArgumentParser(
         description=__doc__,
         formatter_class=argparse.RawDescriptionHelpFormatter,
@@ -2675,6 +2773,7 @@ def main(args):
         action='store', default=None,
         help='the name of the branch to which the result will be stored',
         )
+    add_test_command_argument(subparser)
     subparser.add_argument(
         '--manual',
         action='store_true', default=False,
@@ -2716,6 +2815,7 @@ def main(args):
         action='store', default=None,
         help='the name of the branch to which the result will be stored',
         )
+    add_test_command_argument(subparser)
     subparser.add_argument(
         '--manual',
         action='store_true', default=False,
@@ -2754,6 +2854,7 @@ def main(args):
         action='store', default=None,
         help='the name of the branch to which the result will be stored',
         )
+    add_test_command_argument(subparser)
     subparser.add_argument(
         '--manual',
         action='store_true', default=False,
@@ -2894,6 +2995,7 @@ def main(args):
         action='store', default=None,
         help='the name of the branch to which the result will be stored',
         )
+    add_test_command_argument(subparser)
     subparser.add_argument(
         '--manual',
         action='store_true', default=False,
@@ -3040,9 +3142,14 @@ def main(args):
             tip2, commits2,
             goal=options.goal, manual=options.manual,
             branch=(options.branch or options.name),
+            test_command=options.test_command,
             )
         merge_state.save()
         MergeState.set_default_name(options.name)
+        gcs = GitConfigStore(options.name)
+        if options.test_command is not None:
+            gcs.set(options.name + '.testcommand', options.test_command)
+
     elif options.subcommand == 'start':
         require_clean_work_tree('proceed')
 
@@ -3068,9 +3175,13 @@ def main(args):
             tip2, commits2,
             goal=options.goal, manual=options.manual,
             branch=(options.branch or options.name),
+            test_command=options.test_command,
             )
         merge_state.save()
         MergeState.set_default_name(options.name)
+        gcs = GitConfigStore(options.name)
+        if options.test_command is not None:
+            gcs.set(options.name + '.testcommand', options.test_command)
 
         try:
             merge_state.auto_complete_frontier()
@@ -3126,9 +3237,13 @@ def main(args):
             tip2, commits2,
             goal=options.goal, manual=options.manual,
             branch=options.branch,
+            test_command=options.test_command,
             )
         merge_state.save()
         MergeState.set_default_name(name)
+        gcs = GitConfigStore(options.name)
+        if options.test_command is not None:
+            gcs.set(options.name + '.testcommand', options.test_command)
 
         try:
             merge_state.auto_complete_frontier()
@@ -3188,9 +3303,13 @@ def main(args):
             tip2, commits2,
             goal=options.goal, manual=options.manual,
             branch=options.branch,
+            test_command=options.test_command,
             )
         merge_state.save()
         MergeState.set_default_name(options.name)
+        gcs = GitConfigStore(options.name)
+        if options.test_command is not None:
+            gcs.set(options.name + '.testcommand', options.test_command)
 
         try:
             merge_state.auto_complete_frontier()
@@ -3265,6 +3384,8 @@ def main(args):
             merge_state.save()
         merge_state.simplify(refname, force=options.force)
         MergeState.remove(merge_state.name)
+        gcs = GitConfigStore(merge_state.name)
+        gcs.unset(merge_state.name + '.testcommand')
     elif options.subcommand == 'diagram':
         if not (options.commits or options.frontier):
             options.frontier = True
diff --git a/t/create-test-repo b/t/create-test-repo
index d9e10a4..98230e7 100755
--- a/t/create-test-repo
+++ b/t/create-test-repo
@@ -1,6 +1,7 @@
 #! /bin/sh
 
 set -e
+set -x
 
 DESCRIPTION="git-imerge test repository"
 
@@ -11,6 +12,20 @@ modify() {
     git add "$filename"
 }
 
+modify_buildable() {
+    filename="$1"
+    text="$2"
+    cat > $filename << EOF
+#include<stdio.h>
+
+int main()
+{
+    $text
+}
+EOF
+    git add "$filename"
+}
+
 BASE="$(dirname "$(cd $(dirname "$0") && pwd)")"
 TMP="$BASE/t/tmp"
 
@@ -79,6 +94,42 @@ do
     git commit -m "d$i"
 done
 
+###############
+# Build setup #
+###############
+git checkout -b build master --
+modify_buildable hello.c printf\(\"0\\n\"\)\;
+cat > make_script << EOF
+#/bin/bash -ex
+gcc -o hello hello.c
+EOF
+chmod +x make_script
+git add make_script
+git commit -m "Hello World"
+
+git checkout -b build-left build --
+for i in $(seq 10)
+do
+    modify_buildable hello.c 'printf("'$i' top\n");
+    printf("'$i' bot\n");'
+    git commit -m "build $i"
+done
+
+git checkout -b build-right build --
+modify_buildable hello.c printf\(\"2\\n\"\)\;
+git commit -m "Conflicts"
+modify_buildable hello.c printf\(\"2\\n\"\;
+git commit -m "Conflicts, breaks build"
+modify_buildable hello.c 'printf("'2' top\n");
+    printf("'2' bot\n");
+    typo'
+git commit -m "No conflict, breaks build"
+for i in $(seq 4 6)
+do
+    modify_buildable hello.c printf\(\"$i\\n\"\)\;
+    git commit -m "build $i"
+done
+
 git checkout master --
 
 ln -s ../../imerge.css
diff --git a/t/reset-test-repo b/t/reset-test-repo
index 03645b9..7acf006 100755
--- a/t/reset-test-repo
+++ b/t/reset-test-repo
@@ -20,3 +20,8 @@ do
     git update-ref -d refs/heads/$b
 done
 
+"$GIT_IMERGE" remove --name=build-left-right
+for b in build-left-right
+do
+    git update-ref -d refs/heads/$b
+done
diff --git a/t/test-build b/t/test-build
new file mode 100755
index 0000000..365acf5
--- /dev/null
+++ b/t/test-build
@@ -0,0 +1,60 @@
+#! /bin/sh
+
+# This should be executed in a clean working copy of the test repo.
+
+set -e
+set -x
+
+modify_buildable() {
+    filename="$1"
+    text="$2"
+    cat > $filename << EOF
+#include<stdio.h>
+
+int main()
+{
+    $text
+}
+EOF
+    git add "$filename"
+}
+
+BASE="$(dirname "$(cd $(dirname "$0") && pwd)")"
+TMP="$BASE/t/tmp"
+GIT_IMERGE="$BASE/git-imerge"
+
+cd "$TMP"
+
+# Clean up detritus from possible previous runs of this test:
+git checkout master
+"$GIT_IMERGE" remove --name=build-left-right || true
+for b in build-left-right-full
+do
+    git branch -D $b || true
+done
+
+git checkout build-left
+"$GIT_IMERGE" start --goal=rebase-with-history --first-parent \
+    --test-command="./make_script" \
+    --name=build-left-right --branch=build-left-right-full build-right
+
+# Resolve conflict
+modify_buildable hello.c 'printf("'1' top\n");
+    printf("'1' bot\n");'
+GIT_EDITOR=cat git commit
+"$GIT_IMERGE" continue --no-edit
+# Resolve conflict which breaks build
+modify_buildable hello.c printf\(\"2\\n\"\;
+GIT_EDITOR=cat git commit
+"$GIT_IMERGE" continue --no-edit
+# Resolve the build breakage, no conflict
+modify_buildable hello.c 'printf("'2' top\n");
+    printf("'2' bot\n");'
+GIT_EDITOR=cat git commit
+"$GIT_IMERGE" continue --no-edit
+# modify_buildable hello.c 'printf("'1' top\n");
+#     printf("'1' bot\n");'
+# GIT_EDITOR=cat git commit
+# "$GIT_IMERGE" continue --no-edit
+
+# "$GIT_IMERGE" finish