The Groundwater Replenishment (GWR) system is an innovative approach to keeping Orange County's groundwater basin a reliable source for meeting the region's future potable water needs. A joint program of the Orange County Water District (OCWD) and the Orange County Sanitation District (OCSD), the GWR system is an ambitious, large-scale project that will construct an advanced recycled water treatment facility (AWTF) to treat secondary wastewater for reuse. The facility will replace the aging Water Factory 21 (WF21) which, when completed in 1976, was one of the first recycled water treatment plants in the world. Using microporous membranes as pretreatment for reverse osmosis (RO) and ultraviolet radiation (UV) following RO, Phase 1 of the GWR system will produce 70 million gallons per day (mgd) of highly treated water that will meet all drinking water standards. The treated water will be injected into the ground to supply the local seawater intrusion barrier and percolated into spreading basins for groundwater recharge, thus reducing Southern California's dependence on imported water. Besides providing a new, local source of water, the project will also eliminate the need for an additional outfall to the Pacific Ocean, and will improve the water quality of the Orange County groundwater basin. A project as large and sophisticated as the GWR system faces many significant implementation challenges through both the design and construction phases. The final design phase, which began in March 2001, will be completed by mid 2003. The immensity of the work requires multiple designs with over seven major construction contracts for project facilities, including a temporary pretreatment microfiltration (MF) system, MF and UV pre-selected process systems, the AWTF, the conveyance pipelines, and the barrier facilities. Standards to maintain consistency in design approach and content and a strict quality control procedure have been implemented to provide the foundation for a successful design. Specialized facility teams provide the framework to make and record the approved technical and management decisions guiding the project. Extensive contingency planning that was started during the preliminary design is now being applied to help the project meet strict public health requirements. Combating the negative public perception of drinking recycled wastewater, a cohesive public outreach program encompassing public presentations and print and radio/television media announcements is leading the District's constituents towards ultimate public acceptance of this project where other similar projects in Southern California have failed. Includes tables, figures.