Police generally operate as a reactive force, due to the practical difficulties in discovering crimes which are largely committed in privacy. Strategies used by the Sheffield, England police force include: (1) use of discretion in inferring private criminal behavior from behavior in public, (2) obtaining information from the public, (3) development of an informant network, (4) using information gained from other agencies (for example, monitoring ambulance calls to identify overdosed addicts), (5) extension of the scope of interrogations of suspects, and (6) use of electronic eavesdropping devices. The successful use of these strategies is limited by a shortage of resources, expertise, and time, and by needs to balance efficient law enforcement with the freedom rights of the individual. Notes and over 40 references are included.