In the last decade, a growing portion of the literature on political engagement, social movement and urban studies were interesting by informal modes of engagement, using an astonishing variety of terms to designate it. The main objective of the paper is to propose some conceptual work to better understand what informal participation is or is not. Based on the huge existing empirical works, the first part of the paper will try to understand what is informality, the logic of action behind these unconventional modes of participation and the political context underlying it. The second part will open a discussion on the political qualification of informal participation. If sometimes, informal participation wasnot in fact political ?. This raises the issue of what we are measuring or caracterising when we look at “informal” mode of actions.