Tomales Bay, on the Marin County coast, is one of California's largest and least disturbed estuaries, yet little has been published on the status of its waterbirds or their vulnerability to environmental threats. Shuford et al. (1989) reported the results of waterbird censuses of up to 10 consecutive years in the adjacent Point Reyes area between 1965 and 1982 but provided limited information on waterbird use of Tomales Bay. Other anecdotal accounts and aerial survey work over the last 30 years suggest that Tomales Bay supports unusually high abundances of some waterbird species and is worthy of protection. In this paper, we present results from seven winters of baywide waterbird censuses on Tomales Bay, compare results with other studies to provide a historical and geographical perspective on the importance of Tomales Bay to waterbirds, evaluate species distributions within the bay with regard to possible habitat relationships and the importance of particular habitat areas, and, on the basis of these evaluations, identify needs for conservation of winter waterbird populations on Tomales Bay. We address these objectives with regard to all waterbird species associated with Tomales Bay and immediately adjacent ponds and marshes, with the exception of sandpipers (Scolopacidae), plovers (Charadriidae) and large and medium-sized gulls. Most gulls were excluded from the study because of difficulties in counting large numbers of moving birds and identifying species. Sandpipers, plovers, and gulls were also difficult or impossible to observe from survey boats because these species often are concentrated on exposed tidal flats or in alternative habitats away from the bay. Abundances of winter shorebirds and gulls on Tomales Bay have been reported elsewhere.