diff --git a/lib/Minilisp.pm b/lib/Minilisp.pm
index 7da4a58..88cc55c 100644
--- a/lib/Minilisp.pm
+++ b/lib/Minilisp.pm
@@ -640,6 +640,44 @@ sub macro_cond {
 }
 $macros{cond} = \&macro_cond;
 
+sub macro_when {
+  my $ts = shift;
+
+  my $condition = parser_expr($ts);
+  my $work = parser_expr($ts);
+
+  return sub {
+    my $ctx = shift;
+
+    if ($condition->($ctx))
+      {
+        return $work->($ctx);
+      }
+
+    return undef;
+  }
+}
+$macros{when} = \&macro_when;
+
+sub macro_unless {
+  my $ts = shift;
+
+  my $condition = parser_expr($ts);
+  my $work = parser_expr($ts);
+
+  return sub {
+    my $ctx = shift;
+
+    if (!$condition->($ctx))
+      {
+        return $work->($ctx);
+      }
+
+    return undef;
+  }
+}
+$macros{unless} = \&macro_unless;
+
 sub compile {
   my ($term) = @_;
   my @tokens = tokenize($term);
diff --git a/t/unless.t b/t/unless.t
new file mode 100644
index 0000000..774b7b7
--- /dev/null
+++ b/t/unless.t
@@ -0,0 +1,5 @@
+(expect "unless - true condition"
+        (null (unless t 'fail)))
+
+(expect "unless - false condition"
+        (equal 'ok (unless f 'ok)))
diff --git a/t/when.t b/t/when.t
new file mode 100644
index 0000000..076fcac
--- /dev/null
+++ b/t/when.t
@@ -0,0 +1,5 @@
+(expect "when - true condition"
+        (equal 'ok (when t 'ok)))
+
+(expect "when - false condition"
+        (null (when f 'fail)))