Criminal opportunity theory employs the economic theory of markets to describe and predict how criminals and victims interact. Evidence indicates that potential victims take more self-protective measures when the perceived risk of victimization is greater and that prospective criminals are more likely to attack relatively more vulnerable targets. Little research is available on whether increases in self-protection reduce the total volume of crime or merely displace crime to more vulnerable targets. Benefits of the market perspectives are that (1) it assembles various topics encompassed by criminal opportunity theory into a coherent whole, (2) it is expressed in a form that facilitates borrowing from economic theory, and (3) it generates insights for policy evaluation and criminological theory. A central insight is that law enforcement strategies may alter the quality of opportunities, and thus, precipitate additional crime. Effective incapacitation or rehabilitation policies, for example, may reduce the number of criminals in circulation and thereby reduce perceived risk of victimization. This may result in reduced self-protection measures and consequently, increased crime rates on the part of those criminals who remain active. 59 references. (Author abstract modified)