Participatory design actively involves users throughout the design process, from initial discovery of their needs to the final assessment of the system. As in all forms of interaction design, this requires two complementary processes: generating new ideas that expand the design space, and selecting specific ideas, thus contracting the design space. Interaction designers create prototypes that help them generate and explore this space of possibilities, ideally with the ongoing participation of users. Successful prototypes represent different aspects of the system that highlight specific design questions. We classify prototypes based on: Representation, which refers to their physical form, ranging from paper, cardboard or foam mock-ups to complete software or hardware simulations; Precision, which refers to the level of detail, ranging from simple hand-drawn sketches or physical mock-ups to polished computer-generated images or 3D printed objects; Interactivity, which refers to the level of interaction possible, ranging from no interaction, such as a video scenario, to partial interaction, when a designer "walks through" a scenario, to fully interactive, either simulating interaction with the Wizard-of-Oz method or trying an operational software or hardware prototype; Life cycle, which refers to the expected evolution of the prototype, from "throw away" prototypes in the earliest design phase to successively more developed prototypes in later design phases, to actual components of the final system; and Scope, which refers to the aspects of the final system covered by the prototype, including breadth-first horizontal prototypes, depth-first vertical prototypes, or path-based story prototypes. This chapter explains how to create and use these different kinds of prototypes, with examples selected from key phases of the interaction design process.