The paper discusses the materialities of crowds in distinction to crowd semantics. It contributes to an understanding of the relation between atmospheres/ambiances and the spaces of protesting crowds. It will be argued that the relation has two sides. First, techno-ambiental interventions represent a form of crowd engineering and manipulation. Second, affective atmospheres play an important role within the political spaces of the crowd and facilitate the emergence, diffusion, and stabilisation of protest movements. By drawing on historical and contemporary accounts of crowds, the phenomenologies of Canetti and Schmitz will be combined and meaningful implications for crowd-related research will be addressed.