The principle of autonomy presupposes Patient Decision Making Competence (PDMC). For a few decades a considerable amount of empirical research has been done into PDMC. In this contribution that research is explored. After a short exposition on four qualities involved in PDMC, different approaches to assess PDMC are distinguished, namely a negative and a positive one. In the negative approach the focus is on identifying psychopathologic conditions that impair sound decision making; the positive one attempts to assess whether a patient actually has the required abilities and qualities. Characteristic of the latter approach is the use of (or development of) test-like instruments for PDMC assessment. Some of these tests are discussed and commented on. Although they may be useful in investigating aspects of PDMC, none of the described approaches and tests offers a reliable and valid method for PDMC assessment. In response to a potential misuse of tests, the concept of a “supportive situation” is briefly introduced in order to draw attention to the risk of prematurely deeming patients incompetent on the basis of low test scores, whereas their insufficient performance may be (partly) attributable to alack of situational support. Also, the need for and possibility of an emotionalist concept of PDMC are suggested, as an alternative to the more common rationalist one. In this regard,the legitimacy of competence being conceived as a presumption or fiction of law, deserves further investigation.