The Black Blocs made a spectacular entrance into the Movement for Global Justice on 30 November 1999 at the "Battle of Seattle," when they smashed the windows of McDonald's, Nike, Gap, and a few banks. In April 2009, almost ten years later, a Black Bloc is involved in skirmishes with police at Strasbourg during the NATO Summit. The aim of this article, which is largely based on interviews with militants, analyses of their discourses, and first-hand observations of demonstrations, is to identify how the Black Bloc tactic originated and spread, and to understand the political factors that led activists to adopt it. Three intrinsically political questions are addressed: (1) Who should determine the plan of action within a group of militants? (2) Who should determine the plan of action during a demonstration? (3) Who should determine the criteria to assess the effectiveness of the actions taken by a social movement and speak on its behalf? To answer these questions, the notion of "respect for a diversity of tactics" and the links between the Black Blocs and other militant organizations (both radical and reformist) as well as other blocs (the "Tute Bianche" or White Overalls and the Pink Blocs) are discussed.