The overall aim of the Economics of Sanitation Initiative (ESI) is to promote evidence-based decision making to increase the volume, effectiveness, and sustainability of sanitation expenditure. The evidence produced is useful for a range of stakeholders, including governments at national and subnational levels, donors, nongovernmental organizations, program managers, and the private sector. The present study aims to generate robust evidence on the costs and benefits of sanitation improvements in different programmatic and geographic contexts in Southeast Asia to: Enable explicit comparison of sanitation options on the basis of their relative merits; Identify who might be able or willing to pay for sanitation improvements, and provide further evidence for informing both public and private decisions on the “right” levels of sanitation investment; and Concretely inform planning agencies or service providers on financing requirements over the project or technology life cycle.