Western industrialized countries experienced major reductions in crime for a decade from the early to mid-1990s. The absence of adequate explanation identifies a failing of criminological theory and empirical study. More importantly, it means that none of the forces that reduced crime can confidently be harnessed for policy purposes. Existing hypotheses relating to the crime drops are reviewed and found generally wanting. Many do not stand up to empirical testing. Others do not seem able to explain crime increases (such as phone theft and robbery and Internet-related crimes) that occurred alongside the crime drops. It is suggested that the set of opportunity-related theories, or the criminologies of everyday life, present a more promising line of research. The 'security hypothesis' is discussed wherein changes in the level and quality of security may have been a key driving force3 behind the crime drop, and an agenda of crime-specific research is proposed. Table, notes, and references (Published Abstract)