Asian Americans are the fastest-growing group in the United States, increasing from 0.7 percent in 1970 to nearly 6 percent in 2016. The U.S. Census Bureau projects that by 2065, Asian Americans will constitute 14 percent of the U.S. population. Immigration is fueling this growth: China and India have passed Mexico as the top countries sending immigrants to the United States since 2013. Today, two of three Asian Americans are foreign born—a figure that increases to nearly four of five among Asian American adults. The rise in numbers is accompanied by a rise in diversity: Asian Americans are the most diverse U.S. racial group, comprising twenty-four detailed origins with vastly different migration histories and socioeconomic profiles. In this article, we explain how the unique characteristics of Asian Americans affect their patterns of ethnic and racial self-identification, which, in turn, present challenges for accurately counting this population. We conclude by discussing policy ramifications of our findings, and explain why data disaggregation is a civil rights issue.