Researchers, policy-makers, and practitioners continually attempt to develop and implement strategies that address the onset and continuance of violent behavior in young people. Researchers in multiple disciplines have identified risk factors that predispose young people to later violent offending (e.g., school performance, demographic characteristics). It is now recognized that many youth who have contact with the juvenile justice system also have mental health and/or substance abuse disorders. A study of 2,312 youth examines the relationships between mental health and substance abuse history and violent offending. Mental health and co-occurring disorder histories are related to higher levels of later violent offending, while substance abuse history is related to lower levels of violent offending. Overall, it is difficult to untangle the predictive effects of problem history from those of more traditional criminological risk factors. (Contains 5 tables.)