In western industrial nations, cancer is one of the most frequent somatic diseases showing increasing incidence rates. Although the options for medical treatment and the survival rates for most cancer diagnoses have improved over the last few decades, cancer is still a life-threatening illness associated with psychosocial issues, suffering, and distress. Depending on the severity and duration of symptoms, psychosocial distress due to cancer ranges from normal reactions to psychological comorbidity based on ICD classification criteria. In cancer patients, the most frequent psychological diagnoses are adjustment disorders, anxiety, and depression; prevalence rates in the literature show high variations depending on the tumor type studied and the assessment instrument used. Today, standardized and validated screening instruments and diagnostic interviews are available for the screening and assessment of psychosocial distress and psychiatric comorbidity. The screening of psychosocial distress in cancer patients and the assessment of psychiatric disorders are important tasks of modern cancer treatment in order to determine the need for psychosocial counseling and psychooncological treatment.