The influence of the scientist in the investigation of crime has grown rapidly since the establishment of regional forensic science laboratories about 40 years ago. Eight laboratories with a staff of about 700 now cover the whole of England and Wales and a fictional crime of kidnapping is used to illustrate how the scientist can help the police officer in his enquiries. A broken window with smears of blood on it together with tiny fragments of fibres can reveal a great deal of useful information and the methods of investigation is described; the car the criminals used is defined by an examination of smears of paint on a gatepost, tyre marks in soil and fragments of headlamp lens. The use of computerized information banks dealing with analytical characteristics and their use throughout the country is discussed, together with techniques available for examining traces the criminals may have brought to, or left at the scene. Unsolved problems in forensic science are considered.