To achieve the Millennium Development Goals, all partners (public, private, NGOs) must be engaged for improving and expanding the water supply and sanitation services. Yet, high transaction costs, unclear role allocation and lack of trust and commitment put Private Sector Participation (PSP) at risk. The initiative "Policy Principles and Implementation Guidelines for Private Sector Participation in Sustainable Water Supply and Sanitation" contributes to equitable, effective, ecological and efficient PSP projects. Based on a multi stakeholder process, the Policy Principles are offering an open and transparent framework for the negotiation of valid, widely accepted and action-oriented solutions, while the Implementation Guidelines focus on success factors for building partnerships on the operational level.