It is difficult to say precisely when the business improvement district (BID) was born. BIDs emerged out of legal structures and concepts that date back many decades, but the specific BID form is a relatively recent development. By some accounts, the first BID in the United States was the Downtown Development District of New Orleans, which was established in 1975. Few BIDs were created before 1980, and in most places the surge in BID formation did not really get going until around 19902 — the year that Philadelphia's Center City District was first established. Although new BIDs were created on a regular basis around the country throughout the 1990s and 2000s, it is fair to say that 2010 marks the completion of two decades of what I will call the BID movement — that is, the development and spread of, and the academic and public debate about, this new structure of urban governance. The BID combines public and private, as well as city and neighborhood features, in novel and interesting ways, and it provides a useful means of maintaining and supporting the urban environment. Yet, as many critics have pointed out, the BID also raises troubling issues of urban service inequality, accountability, and the focus of urban governance. It is hard to imagine a better way to analyze the coming of age of the business improvement district than through the extraordinary collection of studies of Philadelphia's BIDs in this issue of the Drexel Law Review.