Suicidal behaviour constitutes one of the most common emergencies in psychiatry. However, its prediction and prevention still represent a very complex and puzzling clinical problem. This lecture deals with the most relevant aspects of suicidal behaviour at the dawn of the new millennium: problems with existing definitions and the need for standardised nomenclatures, international epidemiology of mortality and morbidity (non-fatal suicidal behaviour and ideation), its presently most accredited causality.