From digital fingerprinting to iris and retina recognition, biometric identification systems are a multibillion dollar industry and an integral part of post-9/11 national security strategy. Yet these technologies often fail to work. The scientific literature on their accuracy and reliability documents widespread and frequent technical malfunction. Shoshana Amielle Magnet argues that these systems fail so often because rendering bodies in biometric code falsely assumes that people’s bodies are the same and that individual bodies are stable, or unchanging, over time. By focusing on the moments when biometrics fail, Magnet shows that the technologies work differently, and fail to function more often, on women, people of colour, and people with disabilities. Her assessment emphasizes the state’s use of biometrics to control and classify vulnerable and marginalized populations—including prisoners, welfare recipients, immigrants, and refugees—and to track individuals beyond the nation’s territorial boundaries. When Biometrics Fail is a timely, important contribution to thinking about the security state, surveillance, identity, technology, and human rights.