The concepts of "citizen participation" and "empowerment" are, at present, widely discussed. The authors begin this article by defining these concepts in order to link them to the social development of libraries. Thus, Montréal's public libraries, a knowledge city, adhere to the values associated with citizen participation such as openness, equality, inclusion, and innovation. Their activities help foster the citizen's autonomy and his or her participation in society. Examples of different types of clients will be put forward and the conditions necessary for citizen participation will be outlined. The authors will attempt to assess the impact of citizen participation from a sustainable development point of view. How can public libraries relate and contribute to the cultural component of Agenda 21 from a sustainable development perspective? Why is it relevant to invest and encourage citizen participation in libraries? What gains can be made in the long-term? Examples of social and urban development help further the analysis.