Human–computer interaction (HCI) is a multidisciplinary field that shapes how people engage with digital systems, from everyday tools to transformative technologies. This fourth edition of the Human–Computer Interaction Handbook offers the most comprehensive, globally authored resource available, bringing together over 150 contributors across 59 chapters to chart the current and future landscape of HCI. Now fully updated to reflect the evolving science, practice, and societal relevance of human–technology interaction, the Handbook opens with core insights into human cognition, perception, and motor interaction, then moves through the technologies that mediate experience – haptics, wearables, networked systems, and personal fabrication. It explores the discipline of designing interactive systems, the rigour of evaluation and user research, and the emergence of intelligent, adaptive, and immersive environments. Chapters on social, ethical, and inclusive design foreground the moral imperative of accessibility, cross-cultural sensitivity, and participatory approaches, including co-design with children and neurodivergent users. The volume culminates in domain-specific applications across healthcare, aerospace, digital communities, and civic design, and closes with future-facing reflections on artificial intelligence, neuromorphic interaction, and the evolving professionalism of UX. Every chapter is grounded in real-world examples and case studies, making the Handbook both a scholarly reference and a practical guide. With contributions from leading voices in academia, industry, and government, this edition is essential reading for researchers, educators, and practitioners seeking to understand, shape, and advance the field of HCI.