Impossible Subjects: Illegal Aliens and the Making of Modern America, is a Frederick Jackson Turner Award-winning book by historian Mae M. Ngai published by Princeton University Press in 2004.

The book examines legislation, court cases, and attitudes in the late nineteenth and early twentieth centuries that affected immigration.

Through Ngai's analyses of these factors, readers are shown the long-lasting impacts these cases had on racial categories in the United States, and the way that they were aimed at maintaining whiteness.

Part One deals with the origins of anti-immigration policy and nativism in United States politics during the early 20th century, particularly the use of gradated categories of "whiteness" to permit or deny entry of immigrants from certain European and Asian countries.

Part Two, she focuses on migrants from the Philippines and Mexico in the 1920s by discussing their role in the U.S. economy and how they challenged cultural norms about the traditional work force.