Social movements can have an impact on institutions by aiming explicitly at institutional change or by pursuing policy goals. In the former situation, their chances of success depends on factors internal and external to the movements. In the latter situation, the contribution of policy-oriented movements to institutional reform can be seen as a by-product of their action. A process of pressure by social movements and adaptation by the political authorities who grant the movements formal concessions makes such impact possible. The latter is usually minor in scope and consists of procedural-administrative changes. The examples of the women's movement, which addresses both the institutions and the public policies, and of the antinuclear movement, which usually challenges the public policies, suggests the existence of a process of pressure and adaptation for both types of movements.