This article demonstrates how ethnographic research provides much needed insight into the social and cultural precesses that contributed to the differential undercount of ethnic and racial minorities in the 1990 Census of Population and Housing. The article summarizes and synthesizes data and findings reported in 29 ethnographic reports sponsored by the U.S. Census Bureau and non-profit organizations throughout the country. These reports are based on ethnographic research conducted by qualified ethnographers in 29 sample areas throughout the continental U.S. and Puerto Rico. The article reports that the reasons for the differential undercount of ethnic and racial minorities are varied and complex. Empirically and in the aggregate, there is no single reason why a disproportionate number of ethic and racial minorities were not counted by the 1990 census. Rather, there are a constellation of factors that interact and contribute to the differential census undercount. These factors are listed and discussed in the article. The article concludes with recommendations to the Census Bureau for the improvement and conduct of the year 2000 census.