Using Services to get all the Categories related to an Entry?
If you have an EntryModel and know the category field's handle, you don't really need a service:

$categories = $entry-&gt;categoryFieldHandle;


In other cases, you can construct an ElementCriteriaModel to search for related elements (categories, entries, users or assets), using a number of different variables. Easy example to get related categories from an entry ID:

$criteria = craft()-&gt;elements-&gt;getCriteria(ElementType::Category);
$criteria-&gt;relatedTo = array($entryId);
$categories = craft()-&gt;elements-&gt;findElements($criteria);