In the past, police leaders generally held the view that the investigation of crime required few skills in addition to the training required for general police work. This view was reinforced by research conducted in the late 1970s and early 1980s that concluded crimes were solved because members of the public provided the police with key information in the early stages of the investigation. Absent such information, it is unlikely that the crime will be detected by additional police efforts. The implication drawn from these studies is that because crime-solving depends on information provided by the public, changes in police activity or an increase in resources will make little difference in case outcomes. There are three main reasons why police leaders are now convinced that criminal investigation requires a separate regime of professionalization within the police services. These reasons are changes to the legal framework for criminal investigations under British law, technological and procedural advances in the investigation process, and concerns over police effectiveness and conduct in criminal investigations. These changes require that police investigators acquire additional knowledge of legal parameters for investigations as well as investigative principles and techniques based in research and technological advances in investigative tools, in addition to a range of practical skills in conducting investigations. PIP addresses this need for investigative professionalization by providing a training curriculum for investigators that takes them from the basic levels of investigation to the most complex. This chapter argues, however, that PIP is incomplete because it lacks a capacity to evaluate the operational effectiveness of professional practice. 3 tables, 1 figure, and 83 references