[The placebo effect: classes of explanation]

The placebo effect is a frequent phenomenon in medicine, but very little is known about its mechanisms. An overview is given of the different classes of explanation of the placebo effect in analgesia and in particular the role of endogenous opioids, classical conditioning and expectations. Then the question is raised which are the properties of placebo for which a theory has to provide answers in order to be coherent. These properties are, between others, the efficacy of placebo in a variety of conditions, in individuals with different personality characteristics, etc. Finally, the difficulty of observing individual placebo is emphasized and problems concerning the diagnostic and therapeutic use of placebo are mentioned.