Criminals have a firsthand perspective on why and how to commit crime. In this chapter, we outline and illustrate five ways that offender-based research can be used to inform understanding of crime prevention, more specifically situational crime prevention: namely, (1) by directly determining what works to reduce crime; (2) generating findings that are suggestive of what prevention measures to invent and employ; (3) refining understanding of why a given prevention method reduces crime; (4) figuring out how offenders get around particular prevention measures; and, (5) gathering information on not only the positive but also the unintended, negative outcomes of prevention procedures. We conclude by discussing the choices involved in conducting offender-based research for the betterment of situational crime prevention.