The study of human affective states and their influences has been a research interest in psychology for some time. Fortunately, the presence of an affective computing paradigm allows us to use theories and findings from the discipline of psychology in the representation and development of human affective applications. However, because of the complexity of the subject, it is possible to misunderstand concepts that are shared via human and/or computer communications. With the appearance of technological innovations in our lives, for instance the SemanticWeb and the Web Ontology Language (OWL), there is a stronger need for computers to better understand human affective states and their influences. The use of an ontology can be beneficial in order to represent human affective states and their influences in a machine-understandable format. Truly, ontologies provide powerful tools to make sense of data. Our thesis proposes HASIO, a Human Affective States and their Influences Ontology, designed based on existing psychological theories. HASIO was developed to represent the knowledge that is necessary to model affective states and their influences in a computerized format. It describes the human affective states (Emotion, Mood and Sentiment) and their influences (Personality, Need and Subjective well-being) and conceptualizes their models and recognition methods. HASIO also represents the relationships between affective states and the factors that influence them. We surveyed and analyzed existing ontologies regarding human affective states and their influences to realize the significance and profit of developing our proposed ontology (HASIO). We follow the Methontology approach, a comprehensive engineering methodology for ontology building, to design and build HASIO. An important aspect in determining the ontology scope is Competency Questions (CQs). We configure HASIO CQs by analyzing the resources from psychology theories, available lexicons and existing ontologies. In this thesis, we present the development, modularization and evaluation of HASIO. HASIO can profit from the modularization process by dividing the whole ontology in self-contained modules that are easy to reuse and maintain. The ontology is evaluated through Question Answering system (HASIOQA), a task-based evaluation system, for validation. We design and develop a natural language interface system for this purpose. Moreover, the proposed ontology was evaluated through the Ontology Pitfall Scanner for verification and correctness against several criteria. Furthermore, HASIO was used in sentiment analysis on diffrent Twitter dataset. We designed and developed a tweet polarity calculation algorithm. Additionally, we compare our ontology result with machine learning technique. We demonstrate and highlight the advantage of using ontology in sentiment analysis.