The study of human affective states and their influences has been a research interest in psychology, and is also becoming more and more a field of interest in computer science. The affective computing paradigm allows us to use theories and findings from psychology in the development of human affective applications. However, because of the complexity of human affect, there is a danger of misunderstanding the concepts shared via human or computer communication.We address this matter by using the notion of an Ontology from the Semantic Web field to develop a Human Affective States Ontology (HASO). Our ontology represents the knowledge that is necessary to model affective states and their influencing factors in a computerized format. Our ontology is also extendable to be used in a broader context.