Many countries have sought the involvement of the private sector to upgrade and develop their water and sanitation infrastructure and improve the efficiency of water systems. However, high capital intensity, large initial outlays, long pay-back periods, immobility of assets and low rates of return generate high risks. These factors, when combined with poor initial information and a weak investment environment, constitute important constraints on private sector participation in water and sanitation infrastructure. Recognising this, the OECD has developed practical guidance, building on the OECD Principles for Private Sector Participation in Infrastructure, to help governments and other stakeholders to assess and manage the implications of involving private actors in the financing, development and management of water and sanitation infrastructure. The resulting OECD Checklist for Public Action provides a coherent catalogue of policy directions for consideration by governments, including appropriate allocation of roles, risks and responsibilities, framework conditions and contractual arrangements necessary to make the best of private sector participation and to harness more effectively the capacities of all stakeholders.