Practically speaking, an ontology is just a fancy word for a dictionary. More specifically, an ontology is a way of structuring knowledge, by coding complex concepts into simpler terms. Beyond that, ontologies vary widely in the level of complexity given to structuring the relationships between the terms. Some people use the term ontology in healthcare to refer to only systems that are capable of deeply modeling clinical information. Others use the term to refer generally to all types of abstract health care data sets. Sometimes, people speak about ontologies in terms of codes, code sets, or coding processes. This chapter will cover both important sources of clinical coding systems or ontologies, of several types.