Each year a new eruption of "leaderless" social movements -- from North Africa and the Middle East to Europe, the Americas, and East Asia -- leaves journalists, political analysts, police forces, and governments disoriented and perplexed. Activists too struggle to understand and evaluate the power and effectiveness of horizontal movements. Why have the movements, which address the needs and desires of so many, not been able to achieve lasting change and create a new, more democratic and just society? Some people assume that if only social movements could find new leaders they would return to their earlier glory. Where, they ask, are the new Martin Luther Kings, Rudi Dutschkes, and Steven Bikos? Although today's leaderless and spontaneous political organizations are not sufficient, a return to traditional, centralized forms of political leadership is neither desirable nor possible. Necessary, instead, as Michael Hardt and Antonio Negri argue, is an inversion of the roles of the multitude and leadership in political organizations. Leaders should be confined to short-term, tactical action, while the multitude drives strategy. In other words, the formulation of long-term goals and objectives must come from the collective, rather than designated figureheads. Drawing on the ideas developed through their well-known Empire trilogy, Hardt and Negri have produced, in Assembly, a timely proposal for how current large-scale, horizontal movements can develop collectively the capacities for political strategy and decision-making to effect lasting and democratic change.