Jules Gill-Peterson, a historian of medicine, gender, and sexuality at Johns Hopkins University, presents the first comprehensive history of transgender children in twentieth-century America. Challenging the assumption that transgender childhood is a recent phenomenon, Gill-Peterson traces how medical institutions engaged with gender-nonconforming children from the 1930s onward, revealing a largely forgotten archive of clinical encounters. The book demonstrates that the modern concept of biological sex was itself partially constructed through experiments on children's bodies, particularly intersex children who became subjects for surgical and hormonal interventions. Gill-Peterson excavates how race profoundly shaped medical responses to gender variance, with white children more likely to receive sympathetic treatment while Black children were pathologized or ignored entirely. Drawing on medical records, clinical correspondence, and institutional archives, she shows how transgender children have always existed and have always found ways to assert their identities, even within hostile medical systems. The book argues that contemporary debates about transgender youth would benefit from understanding this complex history, which reveals both the harms of medical gatekeeping and the resilience of young people who insisted on being recognized. Essential reading for anyone seeking to understand the intersection of medicine, childhood, and gender identity in American history.