Sleep medicine and dream science have long been dominated by empirical approaches. Volume 1 of this handbook underscores the critical importance of models and theories of sleep and dreams, which provide the conceptual frameworks needed to interpret empirical findings – spanning biological, psychological, and philosophical perspectives. Featuring contributions from leading experts, chapters examine mechanisms of sleep regulation, functions of sleep and dreaming, and present diverse frameworks side by side, outlining core assumptions, mechanisms, and implications before addressing deeper complexities. Designed for students, researchers, and clinicians across sleep science, neuroscience, psychology, biology, and medicine, the handbook invites critical engagement with models and reflection on how they shape contemporary understanding and research.