Suicidal behaviour is a complex phenomenon that has biological, psychological, social and even environmental roots. Theories of suicide are constructs that attempt to understand an individual’s transition from a “non-suicidal” state to suicidal ideation and attempts. Biological theories focus either on the dysregulation of biological pathways or on the evolutionary roots of suicidal behaviour. Psychological-interpersonal theories attempt to understand suicide in terms of concepts such as psychological pain, hopelessness and cognitive constriction in response to the frustration of normal psychological needs. Sociological theories emphasize the role of culture and social integration on suicidal behaviour, while anthropological perspectives emphasize the uniqueness of a given cultural context and the meaning of suicidal behaviour embedded in that context. Various attempts have been made to reduce these models to certain common factors or dimensions or to integrate them into a higher-order model that provides a bio-psycho-social understanding of suicide. Ecological systems theory offers a promising framework for such an integration, improving cultural and ecological validity and conceptual clarity while fostering inter-disciplinary research into suicide. This chapter highlights the theoretical models that explain suicidal behaviour.