The Government of Tunisia demonstrates increasing commitment to greater private sector participation in the delivery of urban environmental services. The National Sanitation Agency (ONAS'), the main operator in the liquid waste sector, currently entrusts the execution of most studies as well as the construction of new facilities to the private sector. ONAS is now interested in expanding the participation of the private sector to include project operation and financing. The objective of this study is to provide the Performance Monitoring Unit of ONAS with the regulatory means to manage and supervise the private enterprises which will provide liquid waste services in Tunisia. Furthermore, this study will inform the development of a national strategy for private sector participation in environmental services.