The book addresses a central question of criminal procedural law: Is it permissible to deceive the accused during the investigative process? While this topic has mostly been discussed in relation to specific issues, there has so far been a lack of a broad fundamental debate and a monographic treatment. Using the example of so-called "legend" operations, the author approaches the subject from two angles: First, he examines the fundamental permissibility of active deception from a legal dogmatic perspective. Subsequently, he evaluates it in light of the principle of effective prosecution, which is investigated as a procedural principle and also philosophically substantiated in a separate proposal. The author demonstrates that deceiving the accused is generally permissible and may even be required.