Depression constitutes a major health problem. Its prevalence in any one year is about 3 per cent of the general population. Most depressed patients are seen by general practitioners, and represent approximately 10 to 18 per cent of their patients. Depression should be present in about 20 per cent of the inpatients in general medicine departments. Depression and organic pathology are very often associated, thus leading to diagnosis mistakes and delays in medical care. Depressions masked by somatic symptoms frequently provoked such mistakes. Depressions occurring during and after medical illnesses, and more specifically chronic illnesses, are often completely neglected. These depressions hinder the patient's readaptation and impair his recovery although they would, in the majority of cases, be accessible to chemotherapeutic and/or psychotherapeutic treatment.