"What are we doing, and why do we think it's going to make a difference? Are we being effective?" Grantmakers ask evaluation questions like these of their grantees and themselves. This brief guide explains why grantmakers use theories of change to guide their questioning, unearth assumptions that underlie their work, establish common language, and develop strong action plans. Contributors to the guide also describe how a theory of change sets the stage for evaluation by clarifying goals, strategies, and milestones.