Theorists argue that deliberation promotes enlightenment and consensus, but scholars do not know how deliberation affects policy opinions. Using the deliberative democracy and public opinion literatures as a guide, I develop a theory of opinion updating where citizens who deliberate revise their prior beliefs, particularly when they encounter consensual messages. A key aspect of this model is that opinion strength moderates the deliberative opinion change process. In two separate propensity score analyses using panel survey data from a deliberative forum and cross-sectional surveys, I show how deliberation and discussion both affect opinions toward Social Security reform. However, deliberation differs from ordinary discussion in that participants soften strongly held views, encounter different perspectives, and learn readily. Thus, deliberation increases knowledge and alters opinions, but it does so selectively based on the quality and diversity of the messages as well as the willingness of participants to keep an open mind.