We present a general theory about how campaigns can have effects and suggest that the evaluation of communication campaigns must be driven by a theory of effects. The National Youth Anti-Drug Media Campaign illustrates both the theory of campaign effects and implications that theory has for the evaluation design. Often models of effect assume that individual exposure affects cognitions that continue to affect behavior over a short term. Contrarily, effects may operate through social or institutional paths as well as through individual learning, require substantial levels of exposure achieved through multiple channels over time, take time to accumulate detectable change, and affect some members of the audience but not others. Responsive evaluations will choose appropriate units of analysis and comparison groups, data collection schedules sensitive to lagged effects, samples able to detect subgroup effects, and analytic strategies consistent with the theory of effects that guides the campaign.