Improving handwashing with soap practices can save children’s lives by reducing preventable diseases like diarrhea and acute respiratory infections. Despite its effectiveness in reducing disease, handwashing with soap is uncommon in many countries. The Water and Sanitation Program (WSP) conducted research with local partners in Peru, Senegal, Tanzania, and Vietnam to understand the factors that affect an individual’s decision to practice handwashing with soap. The research informed the implementation of handwashing project activities in the four countries. Following national and local government implementation, WSP and its partners gathered valuable lessons, which inform this handwashing with soap toolkit. The toolkit, intended for practitioners interested in behavior change, is organized into four modules, each with reports and presentations about the lessons learned from the projects, as well as mass media, direct consumer contact, and interpersonal communication tools used throughout the project.