Objective: Although widely studied in adults, the link between lifetime adversities and suicidal ideation in youth is poorly understood. The purpose of this study was to explore this link in adolescents. Methods: The analyses used a sample of 740 16-year-old youth in the LONGSCAN sample, and distinguished between childhood (before the age of 12) and adolescent (between age 12 and age 16) adversities. Results: There was a significant link between cumulative lifetime adversities and suicidal ideation. There was no evidence that this link was moderated by gender. Childhood adversities moderated the effects of adolescent adversities on suicidal ideation; effects of adolescent adversities were strongest at low levels of childhood adversities. There was also some evidence supporting a specific cumulative model of the effects of adversities on suicidal ideation; the most predictive model included the sum of the following adversities: childhood physical abuse, childhood neglect, childhood family violence, childhood residential instability, adolescent physical abuse, adolescent sexual abuse, adolescent psychological maltreatment, and adolescent community violence. Conclusion: The timing and nature of adversities are important in understanding youth suicidal ideation risk; in particular, adolescent maltreatment and community violence appear to be strong predictors. Preventing and appropriately responding to the abuse of adolescents has the potential to reduce the risk of suicidal ideation. Copyright 2012 American Psychological Association.