The interpersonal-psychological theory of suicidal behavior (T. E. Joiner, 2005) makes 2 overarching predictions: (a) that perceptions of burdening others and of social alienation combine to instill the desire for death and (b) that individuals will not act on the desire for death unless they have developed the capability to do so. This capability develops through exposure and thus habituation to painful and/or fearsome experiences and is posited by the theory to be necessary for overcoming powerful self-preservation pressures. Two studies tested these predictions. In Study 1, the interaction of (low) family social support (cf. social alienation or low belonging) and feeling that one does not matter (cf. perceived burdensomeness) predicted current suicidal ideation, beyond depression indices. In Study 2, the 3-way interaction among a measure of low belonging, a measure of perceived burdensomeness, and lifetime number of suicide attempts (viewed as a strong predictor of the level of acquired capability for suicide) predicted current suicide attempt (vs. ideation) among a clinical sample of suicidal young adults, again beyond depression indices and other key covariates. Implications for the understanding, treatment, and prevention of suicidal behavior are discussed.