This document discusses the use of ontologies in annotating metadata for the ENCODE project. It describes how ontologies provide a consistent way to describe biological concepts and facilitate identifying related data. The document outlines how biosample, treatment, and assay metadata are annotated using ontologies like Uberon, Cell Ontology, and Experimental Factor Ontology. It also gives examples of how ontologies allow flexible searching of metadata, such as finding all heart-related tissues. Future directions discussed include using additional ontologies and ontology-based validation of data.