I will discuss the type of knowledge generated in participation research, both in terms of its production mode and scientific value. I will argue that Environment-Behavior participatory research has been by and large taught and practiced away from any epistemological paradigm and that this situation may have contributed to its limited dissemination and to the underrating of its value as scientific knowledge. I propose that the way knowledge is built in participation, as well as its resulting integrative nature, are fundamental contributions to the development of post-normal science, and more specifically, of a transdisciplinary research paradigm.Although the concept of participation has been used in planning and architecture since the early 1970s, namely with advocacy planning growing out of a reaction to the urban renewal movement in the 1950s and 1960s, its meaning and use extends far beyond these professions. The renewed intensity given to participation to emerging research fields in the late 1980s, can be related to emerging societal problems and pressure from user groups in relation to peace and conflict research, international cooperation, women’s studies, and nursing care (Elzinga, 2008). Professional workers, often motivated by social movements, asked for their tacit (know-how) knowledge to be integrated in academic research production. As examples of theoretical and methodological outcomes, Elzinga (2008) cites: 1) the concepts of empathy and coping brought by nurses beyond medical knowledge; 2) action research developed by social workers where target groups are integrated in the research at all steps. Elzinga goes on saying that these claims led to the development of innovative research strategies and to the entry of the prefix “trans” to express something beyond “the interaction of different academic tribes as centrepiece”, something transcending disciplinary boundaries.As guest editors of a special issue of the journal Futures on transdisciplinarity, Roderick Lawrence and I (2004) defined transdisciplinary research as projects tackling complex and heterogeneous problems, using non-linear and reflective knowledge production modes, and dealing with local contexts (see also Balsiger, 2004; Klein, 1996, 2004). With no doubts, the concept of transdisciplinarity better fits problem-solving-oriented participatory research than the concept of interdisciplinarity which main aim is to produce scientific knowledge. Moreover, since ethical and aesthetical issues are inevitably imbedded in urban planning and design, they induce a part of uncertainty in the knowledge produced, which is also more readily associated with transdisciplinarity. Finally, transdisciplinary research recognizes the contribution of individuals’ practical reasoning as valuable, which participative and collaborative processes also allow for (Després, Brais, Avellan, 2004). The lack of dissemination of case studies on participation has also contributed to the lack of integration of knowledge in this area of research. Indeed, participation, as a type of applied research most often sponsored, is often left with little or no time and/or money for retroaction and often for publication. Elzinga (2008) criticizes the fact that the literature on participation is largely dominated by descriptive case study reports. In fact, participation theory is underdeveloped, and evaluations of participation methods are rare and often limited to ad hoc suggestions and criticisms about the advantages and disadvantages of various techniques. Indeed, if a fair number of useful handbooks have been published in the last ten years, their main goal is most often to provide toolkits to assist. Despite the exemplary work of several researchers to bring participation in architecture and planning to new analytical levels (Fainstein, 2000; Feldman & Staal, 2004; Forester, 1999; Healy, 1997; Innes & Booher, 2000; Sanoff, 2007; Toker, 2007), we are still is missing a clear framework to compare and contrast the relative merits of participation research, and integrate its resulting forms of knowledge. I believe one reason why this research has been underrated as scientific knowledge is because it has been by and large taught and practiced away from any epistemological paradigm. I propose that the way knowledge is built in participation studies, as well as the integrative nature of the resulting knowledge, are two fundamental contributions to the development of the transdisciplinarity in science that needs to be better acknowledged. Indeed, this recent paradigm might help structure participation as a maturing component of a complex research program on people-environment relations.