People have a fundamental need to belong that, when satisfied, is linked to a variety of indicators of well-being. The current article discusses what happens when social relationships go awry, namely through social exclusion. It seeks to resolve discrepancies in the literature by proposing that responses to social exclusion depend primarily on the prospect of social acceptance. When people feel socially excluded, they want to regain acceptance and thus may respond in ways that can help them do so. When the possibility of acceptance is not forthcoming, however, socially excluded people become selfish and antisocial. Evidence for this pattern was found at behavioral, cognitive, and biological levels. The motivation to gain acceptance may drive people to engage in negative health behaviors, such as smoking. Thus, excluded people demonstrate sensitivity to possible social acceptance, but they can exude an air of selfishness and hostility when there is no possibility of satisfying their need to belong.