This communication argues that participatory/collaborative urban planning and design can be labelled as transdisciplinary research. It discusses some of the theoretical and methodological challenges in order for this area of Environment-Behavior Studies to develop. Indeed, as frequently experienced, this type of applied research is generally left with little or no time and money for theoretical and methodological retroaction. The discussion is structured in three arguments. The first one concerns theoretical development with an examination of the type and value of knowledge produced during participatory processes. The second argument demonstrates the need for some methodological development in terms of innovative strategies and tools. The final argument insists on the need for applying post-facto evaluative procedures to these participatory/collaborative urban planning and design processes. Two case studies of such processes conducted in Quebec City, Canada, are used to illustrate these points. The first one concerns a one year-anda- half process aiming at producing a Planning Development Scheme for Quebec City’s postwar suburbs; the second, describes the process of producing a Master Plan for a sustainable neighborhood on Quebec City's university campus. [SOURCES: 1) DESPRÉS C, FORTIN A, JOERIN F, VACHON G, MORETTI GP, GATTI E (2008) Retrofitting postwar suburbs: A collaborative planning process. In G Hirsch Hadorn, H Hoffmann-Riem, S Biber-Klemm, W Grossenbacher-Mansuy, D Joye, C Pohl, Christian U Wiesmann & E Zemp (eds) Handbook of Transdisciplinary Research. Heidelberg: Springer. 2) DESPRÉS C, BRAIS N, AVELLAN S (2004) Collaborative planning for retrofitting suburbs: Transdisciplinarity and intersubjectivity in action. In RJ Lawrence & C Després (eds) Transdisciplinarity in theory and practice, Futures, 36 (4) : 471- 486. Special issue.; 3) FORTIN A, DESPRES C, VACHON G (2005) Design urbain en collaboration. Bilan et enjeux. In S Lardon (dir) Le diagnostic des territoires, Géocarrefour. Revue de géographie de Lyon, 80 (2) : 145-154. Special issue.; 4) VACHON G, DESPRÉS C, NEMBRINI A, JOERIN F, FORTIN A, MORETTI GP (2007) Collaborative planning and design for a sustainable neighborhood on Quebec City's university campus. In K Thwaites, S Porta, O Romice & M Greaves (eds) Urban Sustainability through Environmental Design, (129-135). London : Routledge.]