The co-occurrence of alcohol and mental health problems in the general population is well-documented. Among young adults, evidence suggests that most of those with an alcohol problem are also suffering with a mental health problem. The transition from childhood to adolescence is a critical point for the development of both alcohol and mental health problems. Young adults who experience alcohol and mental health problems concurrently have worse outcomes than those with one problem alone, and treatments for co-occurring problems are complex. In this chapter, we discuss (1) key theories regarding co-occurring alcohol and mental health problems, (2) evidence on the extent of these issues, and (3) current treatments.