This extensively revised and updated Open Access book offers a comprehensive guide to involving patients in health technology assessment (HTA). Defining patient involvement as research into patient aspects (patients’ experiences, preferences, perspectives, needs) and patient participation in HTA (individual HTAs, methods, processes, governance and policy) it includes detailed explanations of methodologies and approaches, as well as case studies. The book begins by introducing HTA and considering the ethics of patient involvement. It then defines the complementary strands of research and participation before providing chapters on key methods including: qualitative evidence synthesis, patient preference studies, ethnographic fieldwork, social media analysis using AI, natural language processing and deep learning, patient-led collaborative research to inform economic modelling, and evaluation of patient involvement. It concludes with carefully selected case studies highlighting specific approaches and considerations among HTA bodies on five continents, health technology developers and patient organisations. With cohesive contributions from more than 120 authors from a variety of disciplines around the globe and most chapters including patient authors, Patient Involvement in Health Technology Assessment concludes with urgent calls to action for this field which has rapidly gained momentum in the past decade. As both a practical manual and academic text, it serves as a source of inspiration for policymakers, practitioners, researchers, patient advocates and postgraduate students around the world who seek to improve HTA by taking account of patients’ knowledge, experiences, needs and preferences.