The free will debate widely exceeds the neuroscientific and philosophical fields due to profound implications for legislation, case law and psychiatric expert opinion. Data from Benjamin Libet's experiments on the readiness potential have been used as an argument against personal responsibility and for changes in the law. Due to the explicit use of the term “free will” in German civil law, the psychiatrist as an expert witness is confronted with this debate. In this article we outline the role of this crucial term in German civil law and we describe the neurophysiologic challenge in the form of Libet's experiments, which is led on three levels: the correctness of the data, the impact on the question of whether free will exists and possible consequences for the law. We conclude that the problem of free will cannot be debated on the basis of the data provided by Libet's experiments and that doubts about the existence of a free will must not lead to changes in the law or in psychiatric expert testimony. Therefore, advice for the psychiatrist as an expert witness is offered on the basis of a psychopathological approach that takes into account cognitive and motivational preconditions and the structure of values and personality. Copyright © 2007 John Wiley & Sons, Ltd.