Three broad classes of theories about message effects in cancer control are presented in this special supplement to the Journal of Communication. These are behavior change, information processing, and message effects theories. All three types have implications for the design of messages for cancer control. The theories are not just different approaches to a complex problem but offer complementary perspectives on the effects of messages on audiences. This summary article explores why theory is so important to efficient research in message effects and speculates about the interrelation among behavior change, information processing, and message effects models.