Whether looking at the spread and adoption of an intervention across a community, across multiple units, or within a single unit, an understanding of diffusion theory can help evaluators uncover patterns and impacts that might otherwise be overlooked. The theory alerts evaluators to examine why uptake of an intervention appeared different in different sites, according to the characteristics of the people involved, the social systems involved (for example, neighborhoods, states, or organizations), or the communications channels used. Insights might explain intervention intensity across sites and consequent differential effects. It also yields useful information to assist with subsequent replication of the intervention by practitioners and policymakers. © Wiley Periodicals, Inc., and the American Evaluation Association.