Linkage is one of the tools that planners may use in an attempt to address a variety of urban problems, most typically the need for affordable housing. The underlying theory is a simple one: New commercial developments attract new workers to the city, and the new workers require services and facilities that the city must provide; the city in turn looks to the developers who are profiting from the commercial development to share the costs of those services and facilities. If a linkage program is to succeed, it must work economically and it must benefit both the community and the developer. Consequently, a typical linkage program requires certain exactions from the developer in return for certain concessions that the city will make to the developer. In addition to succeeding economically, linkage must be legally defensible. It must be enacted pursuant to an authorized municipal power, and be carefully designed to withstand certain legal challenges that an opponent may raise.