In the Asia and Pacific region, accessing clean water and sanitation is a constant challenge for residents of informal settlements. Informal service providers, or intermediaries, have emerged in response to this significant essential service gap. This publication examines good practices particularly those involving intermediaries—social enterprises, the private sector, and nongovernment organizations—in delivering water and sanitation services in informal settlements, and their potential for expansion. It presents case studies from Bangladesh, Cambodia, India, Nepal, Papua New Guinea, and the Philippines. The learning is intended to encourage governments, water supply and sanitation utilities, and development financing institutions to incorporate intermediary service providers as valuable partners in large-scale, sustainable investments in water and sanitation service delivery.