When a group of individuals combine to form a critical mass, a change that requires collective action can be effected. Individuals must have incentives to act, however, in order for the critical mass to be reached and that is not always the case. A general model of collective action is presented where some arbitrary number K out of N agents must agree to act to change the status quo. Actions are costly and the change affects all N agents. Examples of this setting include public good provisions, political movements, and team production. Despite the free-riding problem and the likelihood that change may never occur, agents are willing to initiate a move or follow early movers if both their private valuations of the change and the estimated probability of its success are high enough. They do this because early actors take into account the impact of their actions on other agents' incentives to participate. Sometimes an insufficient number of agents take early action and the movement peters out, leaving the status quo intact. But in some cases, the collective action takes off and the early actors succeed in creating enough followers. The dynamic pattern of actions is fully characterized.