How can I follow relations in ElementCriteriaModels from plugin code? In Twig code you can use craft-&gt;entries-&gt;relatedTo to do all sorts of neat things. 

From plugin code I am not sure how this works? 

$c = craft()-&gt;elements-&gt;getCriteria(ElementType::Entry);
$c-&gt;section = 'concepts';
$c-&gt;relatedTo(array('targetElement'=&gt;$e, 'field'=&gt;'entry'));
$concepts = $c-&gt;find();


This doesn't throw an error - but it doesn't work either. 

How can I use the criteria model to retrieve entries that have some relationship? 

Cheers!
You're very close! Try this:

$criteria = craft()-&gt;elements-&gt;getCriteria(ElementType::Entry);
$criteria-&gt;section = 'concepts';
$criteria-&gt;limit   = null;
$criteria-&gt;relatedTo = array(
    'targetElement' =&gt; $e,
    'field'         =&gt; $field
);
$elements = craft()-&gt;elements-&gt;findElements($criteria);


Keep in mind, in this instance, $field is the field object which can be retrieved with something like the following:

$singleFieldModel = craft()-&gt;fields-&gt;getFieldById($singleField);


See the documentation (https://craftcms.com/classreference/services/FieldsService) for other ways to get a field object.