The issue of a patient´s competence is often solved in practice without a major theoretical concept. Such an approach, focused only on the basis of intuitions and experiences of physicians, however, may lead in some cases to an inadequate assessment of competence of a particular patient. Ultimately, it can happen that the decision of an incompetent person will be respected or that the decision of the competent person will be disrespected. Both possibilities can have ethical as well as legal consequences. This article addresses both theoretical and practical issues of competence in adult patients.