A range of implantable brain-interfacing devices (IBIDs) is currently in use and development for the treatment of movement disorders and disorders of mood, behaviour and thought. These include cochlear implants, deep brain stimulation (DBS), prosthetic limbs, and optogenetic interventions (the combined use of genetics and optics to control individual cells). While implantable non-brain devices, such as implantable cardioverter defibrillators, began receiving US Food and Drug Administration approval in 1980, the development of IBIDs is recent, with the approval of DBS for Parkinson's disease in 1997. The expansion in use of IBIDs from neurological to psychiatric conditions is even more recent, with current trials underway for a range of disorders including depression, OCD, addiction, Alzheimer's disease and Tourette's syndrome. Emerging applications of existing IBIDs and new devices in development differ from currently approved devices and applications in two potentially crucial ways: 1) They target conditions traditionally seen as psychiatric; and/or 2) They target and modify functions or traits tied closely to agency, personal identity and personhood. As such, understanding patients' and caregivers' conceptions of personal identity in the context of disease and treatment is important not only for the informed consent process, but also for questions of public policy.