The most serious problems standing in the way of developing an adequate theory of computation are as much ontological as they are semantical. It is not that the semantic problems go away; they remain as challenging as ever. It is just that they are joined – on center stage, as it were – by even more demanding problems of ontology.

– Brian Cantwell Smith [1996, p. 14]

How do you represent knowledge about a world to make it easy to acquire, debug, maintain, communicate, share, and reason with that knowledge? This chapter explores flexible methods for storing and reasoning with facts, and knowledge and data sharing using ontologies. As Smith points out, the problems of ontology are central for building intelligent computational agents.