Zend_Markup is currently shipped with one renderer, the HTML renderer. By adding your own tags, you can add your own functionality to the Zend_Markup renderers. With the tag structure, you can add about any functionality you want. From simple tags, to complicated tag structures. A simple example for a 'foo' tag: addTag( 'foo', Zend_Markup_Renderer_RendererAbstract::TYPE_REPLACE | Zend_Markup_Renderer_RendererAbstract::TAG_NORMAL, array( 'start' => '-bar-', 'end' => '-baz-', 'group' => 'inline', ) ); // now, this will output: 'my -bar-tag-baz-' echo $bbcode->render('my [foo]tag[/foo]'); ]]> Please note that creating your own tags only makes sense when your parser also supports it with a tag structure. Currently, only BBCode supports this. Textile doesn't have support for custom tags.