This article focuses on the so-called “witness test（Shounin Tesuto）”, or preparation of witnesses by attorneys, especially prosecutors. Preparation of witnesses is said to have become an essential part of the adversary system in Japan. However, in recent years, there have been a considerable number of cases where prosecutors were reported to have induced or coached the testimony of witnesses during this preparation process. The United States has a long tradition of preparing witnesses, but the “coaching” of witnesses is said to be unethical. This article summarizes the current situation and ongoing debate in the U.S. It argues for the need to establish a bright-line rule on witness preparation.