Suicidal behavior comprises a range of heterogeneous entities, from completed suicide to suicide attempts to suicidal ideation (Mann 1998). While the causes of suicide are complex and no simple explanations of the phenomenon exist, it is clear that there is a diathesis component, whereby converging factor such as an acute stressor as well as present and past life circumstances can operate on a backdrop of biological susceptibility. This chapter summarizes the principal research addressing genetic susceptibility to suicide. Even if only the biological vulnerability can be explained, it is acknowledged that the origin and causes of suicidal behavior are a multifactorial act, in which biological aspects are always related to the influence of the environment.