Along with psychosocial factors of suicide, biological backgrounds of suicide are explored by extensive works mostly on biological markers, neurobiological models, genetic bases, and relationships with aggression and violence. The biology of suicide confers on neurotransmitters in central nervous system exploring metabolites, receptor binding affinities, neuroendocrine challenge tests in brain, cerebrospinal fluid, blood and etc. The major concerns with suicide are focused mainly on serotonin system : low CSF 5-HIAA concentration, higher $5-HT_2$ receptor binding, and blunt prolactin response to fenfluramine. Postmortem study, in vivo study, genetic contributions, and some other issues such as suicidal methods, serum cholesterol, alcohol, and selective serotonin reuptake inhibitors are reviewed and discussed.