This paper examines the participation rate of the earned income tax credit (EITC). After examining a variety of data sources on EITC recipiency, my preferred estimates indicate that 80 to 86 percent of eligible taxpayers received the credit in 1990, which implies fewer than 2.1 million taxpayers entitled to the credit failed to receive it. I then examine factors correlated with nonparticipation and find that many are consistent with rational or voluntary explanations for nonparticipation. The paper concludes with a discussion of the labor market incentives and antipoverty effectiveness of the credit before and after the August 1993 expansion of the EITC.