Scientifically, compliance can be expressed as the ratio between an observed treatment behavior and given treatment standards. Although the factors that influence compliance often overlap or influence each other, it is possible to differentiate between factors that are related to the patient, the patient's environment, the treating clinician, and treatment itself. Although this differentiation is in some ways artificial, it may aid the practicing clinician in assessing the various reasons why a particular patient is likely to develop or has already developed compliance problems. The fact that a number of circumstances and parties are involved in the multifaceted issue of compliance makes it evident that patients must never be solely blamed for compliance problems. Despite the fact that compliance behavior is difficult to study from a methodological point of view, the scientific information available to date does provide many important leads for the engaged clinician to prevent and manage compliance problems.