Land use changes and development practices that increase imperviousness can significantly alter and disrupt the natural and water resources within watersheds. Evaluating impacts of imperviousness in large watersheds can be a labor-intensive and time-consuming task and there is a need for simplified, planning-level methods that are easy to use and apply. There is also a need to educate and communicate results at the community level. This chapter presents methods for estimating the increase in impervious cover from development and for quantifying the resulting impact of various development practices on imperviousness in large watersheds. A case study in northeast Ohio demonstrates the approaches and methodologies.