This book provides a theoretical framework for design researchers interested in developing their research work about and with prototypes. It describes prototypes' multiple definitions and possible functions in the multifaceted landscape of today's design research. Indeed, while the manufacturing and digital evolutions enable and require new types of advanced prototypes, the design evolution as an academic discipline continuously widens its perspectives, fields, and research methods, among which are the prototypes' applications, features, and purposes. The framework originates from an interpretation of these interrelated phenomena and a collection of case studies. Additionally, it defines those criteria that characterize the relation between the research (context, discipline, and general scope) and the prototype (nature and specific aims) and, altogether, it presents an overview of prototypes' roles in design research.