Abstract
In this paper we elaborate on the scalability of Participatory Design (PD) with a special attention towards engagement. We ground our reasoning on the concepts of scaffolding and infrastructuring as instruments for engaging large groups of heterogeneous participants. Scalability is defined with reference to both the number of people and the space for active participation. We suggest that socio-technical scaffolding is needed to enable the infrastructuring process. Scaffolding and infrastructuring, meant as consecutive phases of design, can enact and support the scaling up of engagement.