This issue of Sleep Medicine Clinics, Guest Edited by Teofilo Lee-Chiong MD, will focus on Biology of Sleep, with article topics including: Neurology of Sleep; Normal Sleep and Circadian Rhythms: Neurobiologic Mechanisms Underlying Sleep and Wakefulness; Physiology of Sleep Disordered Breathing; Evaluation of the Patient Who Has Sleep Complaints: A Case-Based Method Using the Sleep Process Matrix; Sleep States,Memory Processing, and Dreams; Neurobiologic Mechanisms in Chronic Insomnia; Sleep Behavior and Sleep Regulation from Infancy through Adolescence: Normative Aspects; Biological Timekeeping; Neurologic Basis of Sleep Breathing Disorders; Genetics of Sleep Timing, Duration, and Homeostasis in Humans; Sleep in Normal Aging; Modulation of Endocrine Function by Sleep-Wake Homeostasis and Circadian Rhythmicity; Cardiac Activity and Sympathovagal Balance During Sleep; Sleep and Cytokines; Neuropharmacology of Sleep and Wakefulness; Staging Sleep; Respiratory Physiology During Sleep; and Cyclic Alternating Pattern (CAP), Sleep Disordered Breathing, and Automatic Analysis.