Many aspects of modern life raise questions about individual identity: how we prove that we are who we say we are, and how we can be sure that another person is who he or she claims to be. ‘Are you who you say you are?’ considers the different ways of establishing personal identity and the fundamentals of biometrics. In principle, any human characteristic can be used as a biometric data source provided it meets the following four basic criteria: universality, uniqueness, permanence, and collectability. The source of the measurement data used for identification—biometric modality—can be either physiological or behavioural. Some applications of biometric systems are also considered.