User Labs (such as Usability Labs, Living Labs, or Makerspaces) mark a shift in innovation from expert-driven, closed systems to participatory, technoscientific practices by involving laypeople, testing social as well as technical innovations, and treating prototypes as tools for both knowledge production and societal transformation through material experimentation. User Labs are highly heterogeneous. Their commonality lies in trying to expand the social dimension of innovating—by including new groups of actors who were previously only addressed as consumers and users. These are involved in the construction, evaluation, and demonstration of prototypes. The article first describes the expectations of this social opening. Then, various types of User Labs are introduced. The chapter concludes with considerations of the socialization of prototyping, which is expressed in these labs, and the tensions that may arise. These concern the role attributed to users, the ways in which Labs are established, the handling of the intellectual property of generated ideas and products, as well as the question of whether the objectives associated with participatory innovation processes are achieved.