Lithium used as a drug treatment for major mental disorders such as bipolar disorder and depression is effective in reducing the risk of both attempted and completed suicide. However, the mechanisms underlying lithium's antisuicidal actions are not yet known, limiting the development of novel lithium-mimetic compounds that may help reduce suicide risk with fewer undesirable side effects. Suicide is a complex behavior, complicated to study in humans, and impossible to fully reproduce in animal models. The endophenotype approach, by which quantitative measures of neurobiological function are used to assess and subclassify psychiatric illness, may present a path to new discoveries. Aggression and impulsivity are candidate endophenotypes strongly associated with suicide; we review the evidence supporting aggression and impulsivity as suicide endophenotypes, as well as the effects of lithium on these constructs in both humans and rodents. Examining the mechanisms that contribute to lithium's antiaggressive and antiimpulsive effects may assist in understanding how lithium acts to reduce the risk of suicide and in elucidating the neurobiological underpinnings of suicidal behavior.