A comprehensive model of suicidal processes and behavior is essential for the assessment of imminent risk for suicide and for the design of informed interventions. This chapter provides descriptions of the three generations of the most influential theories of suicidal behavior as well as an assessment of their strengths and limitations. First-generation models were based on clinicians’ individual experiences and, more recently, on consensus opinion and clinical judgment. Second-generation prognostic models hypothesized that suicide risk was determined by measurable long-term biological, clinical, or demographic risk factors. Third-generation models of suicidal behavior focused on dynamic risk elements, which appear later in life, change over time, and are operational immediately proximal to suicide. This chapter provides a historical perspective on the evolution of the theoretical approaches to the understanding of psychological processes that make suicide possible.