The purpose of this guide is to provide a process that communities can use to assess the costs and benefits of green infrastructure to reduce flooding. The guide takes a step-by-step, watershed-based approach to documenting the costs of flooding; projecting increased flooding and associated costs under future land use and climate conditions; and calculating benefits and costs of reducing flooding with green infrastructure over the long term. The guide is based on the experiences of three communities in the Great Lakes region (Duluth, Minnesota; Toledo, Ohio; and Green Bay, Wisconsin). To ensure transferability to different types of watersheds and communities, the methods were applied to the He'eia watershed in Hawai'i. Valuable transferability insights from the He'eia watershed partnershave been incorporated.