Medical or clinical practice guidelines of professional institutions establish rules (standards) for good medical treatment (quality of care). They take the form of international, national, regional and local guidelines and are set up by international and national medical organisations, mainly by medical scientific societies, but also by regional (e.g. medical associations in the German Länder states) and local (up to hospital departments) institutions. The rules of good medical treatment (medical or clinical practice guidelines) have to conform to the corresponding medical standard, which is defined by scientific knowledge (evidence-based medicine), practical experience and professional acceptance. The binding character of the standard for medical practice increases with the reliability of scientific evidence and practical experience. Insofar, medical guidelines which correspond to the standard fulfil a quality assurance function for medical treatment as a means of communication in the medical profession, an implementation function for the enforcement of standards and thus also a protective function for patients. Three main issues are concerned: the harmonisation of the procedure for development and the criteria for the quality of good clinical guidelines ("guideline for guidelines"); the legitimation of the objectives of professional guidelines towards the patients and the transformation or reception of medical guidelines by the law. Guidelines can be legally transformed by the courts into standards of due care in liability law, into statutory health insurance law (directives) by the legislator or into the law of tax-financed national health systems. In this function they rationalise (e. g. medical malpractice) decisions of the courts or the legislator by increasing the transparency of their medical basis and thus also contribute to the quality improvement of legal decision-making on medical malpractice. Medical guidelines can also influence (legally binding) health insurance law directives. On principle, guidelines of the scientific medical associations do not involve any economic considerations (efficiency) and should be strictly separated from these (principle of separation and transparency). Medical guidelines have to be differentiated from (health insurance law) directives and recommendations. In addition to the quality assurance function, the guarantee function for efficiency of treatment is a priority for health insurances.