American government officials experimented with a variety of tools for public administration in the early twentieth century. The regulatory commission became the best known of these new institutional forms, but another Progressive Era innovation with profound and ambiguous implications for U.S. political development was the government-sponsored corporation. Often called “public corporations,” these instrumentalities were created to carry out public purposes, but they were established as separate legal entities to function outside the standard departmental structure of government (and its organizational principles and restrictions). Today, these structures are most prolific at the state and municipal level, where they are generally termed “public authorities.” Since World War II they have been the fastest growing kind of government unit, with, at present, around ten thousand in existence. While everyone perceives these institutions as important players in local affairs, even well-informed citizens are frequently puzzled when it comes to knowing exactly what they are or what they do.