The demonstration project can be an effective organizational form to transform a clean technology prototype - for example, in the field of photovoltaics, carbon capture and storage, or wind power - into a marketable product. A question with regard to the clean technology demonstration project is how its effectiveness can be increased. This article answers this question by reviewing scientific literature on clean technology demonstration projects of the past 39 years. It distinguishes and analyzes three types of demonstration projects: the technical demonstration, organizational demonstration, and market demonstration. The article proposes that the effectiveness of demonstration projects increases when clean technology prototypes are tested, improved, and marketed successively. First, they should be tested in technical demonstrations, then improved in organizational demonstrations, and finally marketed in market demonstrations. The article also proposes five managerial factors that stimulate the effectiveness of the three consecutive demonstration types. The article concludes with an agenda for future research, concentrating on the diffusion of clean technological innovations through a coherent program of demonstration projects. It presents several theoretical approaches that can be applied to conduct future demonstration project research.