An important aspect of ontology learning is a proper evaluation. Generally, one can distinguish between two scenarios: (i) quality assurance during an ontology engineering project in which also ontology learning techniques may be used and (ii) evaluating and comparing ontology learning algorithms in the laboratory during their development. This paper gives an overview of different evaluation approaches and matches them against the requirements of the scenarios. It will be shown that different evaluation approaches have to be applied depending on the scenario. Special attention will be paid to the second scenario and the gold standard based evaluation of ontology learning for which concrete measures for the lexical and taxonomic layer will be presented.