This introduction to the topic of ontology - the philosophical study of what exists and what it means for something to exist - aims to provide students taking courses in metaphysics with a comprehensive account of the central ideas at the heart of the subject of being. The first part of the work explores questions of pure philosophical ontology: what is meant by the concept of being, why something exists rather than nothing, and why there is only one logically contingent actual world. The author argues that logic provides the only possible answers to these fundamental problems of pure ontology. The second part of the book examines issues of applied scientific ontology. The author offers a critical survey of some of the most influential traditional ontologies such as the distinction between appearance and reality, and the categories of substance and transcendence. The ontology of physical entities - space, time, matter and causation - are examined as well as the ontology of abstract entities - sets, numbers, properties, relations and propositions. The special problems posed by the subjectivity of mind, and of God, are also explored. The final chapter deals with the ontology of culture, language and art.