This chapter considers the why, what, when and how of 'evidence-based policing'. It explores other fields where the use of evidence has been crucial to their success, most notably medicine and engineering. The chapter reviews why we need to make more use of evidence in policing. It outlines several differing kinds of evidence that have a role to play, making the point that the evidence which is now being emphasised is not 'injected' into evidence vacuum. The chapter discusses when and how evidence of different kinds can be most usefully injected into policing. It suggests that there is scope for greater use of evidence at diverse points in the education and training of police officers, in the conduct of their work, in original research and in reviewing and learning from failures. The chapter discusses parallels between the use of evidence in health care and engineering and that which could enhance policing.