The issue of density and of one of its primary regulatory tool, the FAR (Floor Area Ratio), has given rise to extensive and controversial debates when Paris conducted its local urban plan review process from 2001 to 2006. This paper explores these debates by focusing on the key actors involved, their points of view and motives and the spaces for deliberation and decision-making. Although it is criticized severely and is considered by many to be inefficient from a planning standpoint, the FAR shows a surprising capacity to withstand change and plays a major role in framing public debates on how to manage density. This great “career” is due in part to its operational role in strengthening the building market, but it is also assured by the symbolic and political significance it carries. Since the late 1960’s when it was first introduced, the FAR has conveyed an important part of Parisian urban planning history and a particular political and regulatory dynamics, so that any amendment, despite being supported by some experts and local officials, is still highly uncertain today.