Basic subpage listing in WordPress

list-subpages.php

<?php

// List subpages on parent and child page. Outputs ul > li.

$child_of = ($post->post_parent > 0) ? $post->post_parent : $post->ID;

if ($child_of > 0 && is_page()) {
  wp_list_pages('title_li=&child_of='.$child_of);
}