This innovative book, written by an author with borderline experiences of her own, explores the lived experience known as “borderline personality disorder”. It draws on neuroqueer approaches and feminist new materialist philosophy to highlight the potential of the unstable knowledges produced in the borderland. Resisting either embracing the clinical definition of borderline personality disorder or rejecting the diagnosis as a misogynistic construct, this book focuses on the ontological aspects of borderline experience, exploring unstable and indeterminate thoughts, feelings, perceptions, and selves. It illuminates how paying attention to these unique ways of seeing and knowing can provide alternatives to the ‘cruel optimism’ of medicalised ways of knowing BPD and challenge the harmful dualistic thinking that reinforces them. Demonstrating how we might understand borderline experience as a diagnostic, philosophical, or mystical mode for knowing self and world through unique orientations in time, space, and reality, this book is an important contribution to the medical and health humanities and critical mental health. It simultaneously provides a fresh new way of thinking about the borderline and offers new critical perspectives and interventions for the wider landscapes of disability studies, medical humanities, and justice-oriented disciplines. It will be of use to a range of disciplines and professions with an interest in mental health, including healthcare, psychology, sociology, philosophy, anthropology and disability studies.