Based on the literature, the assessment of psychological comorbidity in epidemiological studies is reviewed along with prevalence rates of psychological comorbidity and the effect of mental diseases on the development and course of chronic disease as exemplified by coronary heart disease. Psychological comorbidity is associated with reduced quality of life, disease progression, and increased mortality, as numerous studies in coronary heart disease and other chronic diseases have shown. In spite of available valid screening measures which enable large population surveys, diagnostic problems remain, especially with respect to separating mental and somatic symptoms and diagnosing somatic diseases reliably. Psychological comorbidity is frequently overlooked in medical care and poses a high risk for the future somatic and mental course of disease, associated with individual suffering and serious health economic consequences. There is a need for research on suitable interventions for patients with chronic disorders and psychological comorbidity in order to improve their care.