Editors' Summary: Beginning with Agins v. City of Tiburon and continuing for 25 years, the U.S. Supreme Court has held that regulation effects a taking when it does not substantially advance legitimate state interests. Throughout this period, many have criticized this standard as "a return to Lochner," opposed to the extreme deference accorded economic and property regulation since the New Deal. According to this author, however, a careful review of cases reveals that the "substantial advancement" doctrine is not simply a means-ends review of the efficacy of economic legislation. Rather, the doctrine was initially conceived, and has been applied, as a cause and effect test to ensure that restrictive land use regulations are designed to mitigate social costs that would be caused by the unregulated use of the property in question. Although no return to Lochner, in some cases (most recently in Lingle v. Chevron) the doctrine confronts the need to set limits to the proper exercise of the police power--a function that has been abdicated by the judiciary since Nebbia v. New York. This deeper conflict explains the vehemence of Agins' critics and, the Article concludes, must be resolved if takings law is to shed its post-New Deal ambiguity and function effectively in the unending struggle of constitutional principle against legislative will.