This chapter describes the contradictory roles that weapons play in offender decision making as mechanisms that can both increase the physical harm to a victim of violence and also reduce the need for physical harm in victims of robbery. Because weapons serve simultaneously offensive and defensive purposes, the way in which offenders carry and use weapons is subject to a complex decision-making process. This process is presented and interpreted from a rational perspective, incorporating an offender’s calculation of potential benefits and costs as well as the uncertainty of a victim’s response. A rational analysis of weapon carrying and use is presented along with research evidence suggesting that culture and availability are important influences on weapon of choice and weapon-related behavior. The chapter concludes with a review of the effectiveness of weapons in reducing victim resistance and retaliation showing that weapon use is a high-reward/high-cost activity.