A newly produced (2004) Combined Sewer Overflow (CSO) Treatment Technologies Manual provides an overview of practices in, and guidance for, planning, design and implementation of CSO treatment. Towards this end, the manual first addresses CSO abatement program planning in order to provide an overall framework within which the application and design of CSO treatment facilities may be considered. Following the establishment of a planning framework, guidance is provided for CSO treatment facility preliminary design, which encompasses such steps as the development of CSO control and treatment objectives, assessment of design flows, development of a treatment process train, site-specific considerations, and costs and operating and maintenance (O&M) factors. Finally, preliminary design considerations are developed in more detail for CSO retention treatment basins (RTBs), which are commonly applied to control and treat CSOs in the Great Lakes municipalities. Well-designed RTBs can meet the Ontario requirements for CSO pollution abatement and thereby contribute to the remedial action in the Great Lakes Areas of Concern.