Traditionally, police effectiveness has been measured by data bearing upon crime prevention and the detection and punishment of offenders. The effect of policing upon crime levels over a broad geographical area for an extended period of time cannot be shown to be significant. Crime levels can be shown to be related to many factors other than policing, such as the level of unemployment in an area and the general social environment of the community. Further, determinations of the crime level are elusive, since many crimes are committed of which the police are never aware. Neither can police effectiveness be measured solely by the degree of success in clearing up reported crime. To discuss clearance rates for total crime is of little or no value in assessing police effectiveness, because the overall figure masks wide variations between different types of crime. Crime levels and clearance rates can be helpful in revealing the prevalence of certain types of crime which may need to be targeted by police and also point up strengths and weaknesses in police investigation procedures, but by themselves, they cannot be primary measures of police effectiveness. British policing is based upon a supportive relationship between the police and the public. The police do not control the community by force, but rather by the consent of the residents. The effectiveness of the police in using their discretion and human relations skills to foster community support for their presence and activity should therefore be an important measure of their success in Great Britain. Twenty footnotes are listed.