Modern urban planning began because of the need to create a more sanitary urban environment by demolishing the worst slums and controlling the quality of new construction. Effective urban planing is made up of clear development objectives and a means to carry them out. Objectives vary from community to community, and even within a community. A city may choose to develop its unbuilt land, revitalize its downtown areas, demolish its slums and replace them with new buildings, or any combination thereof. In this country, direct intervention by the government was the key to urban renewal. Such intervention can range from eliminating substandard housing to the planning of new towns from the bottom up.