Evaluation is a very vital research interest in the digital library domain. This has been exhibited by the growth of the literature in the main conferences and journal papers. However it is very difficult for one to navigate in this extended corpus. For these reasons the DiLEO ontology has been developed in order to assist the exploration of important concepts and the discovery of trends in the evaluation of digital libraries. DiLEO is a domain ontology, which aims to conceptualize the DL evaluation domain by correlating its key entities and provide reasoning paths that support the design of evaluation experiments.