Treatment manuals are increasingly being used to guide interventions with individuals, families, groups, organizations, and communities. However, little is known about best practices in designing manuals. We describe a process that provides for the development of manuals and specifies the means by which manuals can be adapted for practice conditions and constraints. Manual development is conceptualized as comprising four systematic and recursive stages: (a) formulation, (b) revision, (c) differentiation, and (d) translation. We discuss issues and challenges in developing manuals that are responsive to a range of factors that influence social work practice, including advances in knowledge, the influence of evidence-based practice, the needs of individual clients, and contingencies linked to organizational policies, procedures, and leadership. (Contains 2 figures.)