We analyzed patterns of abundance and distribution of wintering waterbirds in Tomales Bay, California, based on 21 baywide winter surveys over 7 years from 1989-90 to 1995-96. Total waterbird abundances averaged 21,943 (min. = 14,842, max. = 25,553), excluding shorebirds and gulls. Mean waterbird density was 770 birds/km2 with the greatest concentrations along the east shore and in the area of the Bay between Pelican Point and Tom’s Point. Surf Scoter, Bufflehead, and Greater Scaup together accounted for 70% of the total waterbirds. Aerial count data from the U. S. Fish and Wildlife Service Midwinter Waterfowl Survey underestimated the numbers of waterbirds compared to our counts made from boats. Adjusting for such differences, our abundance estimates for scoters and Bufflehead appear similar to other abundances reported in the 1960s and 1970s. Compared to other coastal wetlands in California, Tomales Bay provides particularly important winter habitat for Red-throated Loons, Common Loons, Eared Grebes, Horned Grebes, Black Brant, Surf Scoters, and Black Scoters. With the exception of San Francisco Bay, Tomales Bay may provide the most important winter habitat for Bufflehead on the Pacific Coast south of the Columbia River. Waterbird distributions on Tomales Bay are associated with a dynamic array of habitat conditions. Increasing human activities in Tomales Bay and its watershed interact with these processes and conditions, and suggest several concerns for conservation of winter waterbirds. Protection of waterbird populations will require particular attention to the control of human disturbance, protection of eelgrass beds, the possible effects of the herring fishery and aquaculture, and management of processes in the watershed.