This chapter outlines the development and validation of the five-step Bystander Intervention Model, including a visual representation of the process. It begins with a historical overview of bystander behavior research, spurred by the 1964 murder of Kitty Genovese. Experimental studies conducted by Bibb Latané and John Darley that discovered a bystander effect, where individuals are less likely to help when others are present, are described. The bystander intervention model, which includes a series of decisions, or cognitive and behavioral processes, that an individual must make when intervening in emergency situations, is explained. The model’s application to various contexts, including bullying and sexual harassment, is described. This chapter also introduces a 16-item self-report measure designed to assess individuals’ intentions to engage in each of the five steps in bullying and sexual harassment scenarios. Studies that support the measure’s factor structure, convergent validity, and consistency across gender are also summarized.