Where there is data there are questions of ownership, leaks, and worries about misuse. when what's at stake is data on our brains, the stakes are high. this book brings together philosophical analysis and neuroscientific insights to develop an account of 'brain data': what it is, how it is used, and how we ought to take care of it. emerging trends in neuroscience appear to make mental activity legible, through sophisticated processing of signals recorded from the brain. this can include artificial intelligence (ai), with algorithms classifying brain signals for further processing. these developments will have ramifications for concepts of the brain, the self, and the mind. they will also affect clinical practices like psychiatry, by modifying concepts of mental health and introducing ai-based diagnostic and treatment strategies. the issues arising are vastly complicated, little understood, but of high importance. philosophical perspectives on brain data clarifies complex intersections of philosophical and neuroscientific interest, presenting an account of brain data that is comprehensible. this account can be the basis for evaluating practices based on brain data. as such, the book aims to open a novel space for evaluating hitherto arcane areas of academic research in order to provide the necessary scope for understanding their real-world consequences. these consequences will include personal, socio-political, and public health dimensions. it is therefore vital that they are understood if their impacts upon aspects of everyday life can be evaluated adequately. stephen rainey is a lecturer in the philosophy of technology at technische universiteit, delft, netherlands. his recent work has focussed on ethical and epistemic issues concerning brain computer interfaces, including how these impact upon identity, rationality, and control over actions.