Interest in, and research on, comorbidity has been increasing over past decades. It has emerged as one of the complex issues facing theorists, clinicians and policymakers who are responsible for providing funding for mental health problems. This chapter has discussed the concept of comorbidity and discussed the importance of community samples in documenting the extent of comorbidity in the community. As will be documented in later chapters, these surveys have established that comorbidity occurs. This has implications for theory, prevention and treatment of mental health problems. There are a number of potential explanations for comorbidity.