The diencephalon represents a critical division of the forebrain, positioned between the brainstem and cerebral hemispheres. This complex structure serves as a central relay station for sensory and motor information, regulates autonomic functions, and maintains homeostatic control. The diencephalon comprises four major subdivisions: the thalamus, hypothalamus, epithalamus, and subthalamus. The thalamus functions as the brain's primary sensory relay center, processing and directing information to appropriate cortical regions while also integrating motor signals from the basal ganglia and cerebellum. The hypothalamus orchestrates vital autonomic and endocrine functions, including temperature regulation, circadian rhythms, hunger, thirst, and pituitary hormone control. The epithalamus, containing the pineal gland and habenular nuclei, regulates circadian rhythms through melatonin secretion and processes emotional and reward-related information. The subthalamus, primarily the subthalamic nucleus, plays an essential role in motor control as part of the basal ganglia circuitry. Vascular supply to the diencephalon derives mainly from branches of the posterior cerebral and internal carotid arteries. Understanding diencephalic anatomy is fundamental for medical professionals, as dysfunction in these structures underlies numerous neurological and psychiatric conditions, including Parkinson's disease, Wernicke-Korsakoff syndrome, and various endocrine disorders. This chapter provides comprehensive coverage of diencephalic neuroanatomy, including detailed nuclear organization, connectivity patterns, functional correlations, and clinical significance.