This article introduces the special issue of Journal of Personality on personality and problem behavior. There is growing evidence that various dimensions of personality are associated with a broad range of problem behaviors such as drug and alcohol use, sexual risk-taking, interpersonal violence, and gambling. To date, research on personality and problem behavior has focused almost exclusively on documenting simple, direct associations between personality traits and problem behaviors. This article proposes an expanded research agenda, a second generation of research and theory on the role of personality in problem behavior. The agenda specifies two major classes of second-generation hypotheses. Moderator hypotheses concern the conditions under which the influence of personality on problem behavior is magnified, weakened, or eliminated. Mediator hypotheses concern the causal processes that underlie the influence of personality on problem behavior. Articles in the special issue, which are summarized in this introduction, exemplify the proposed second generation of research on personality and problem behavior.