This module provides a comprehensive introduction to the endocrine system and the role of hormones as chemical messengers regulating homeostasis, metabolism, stress, mood, sleep, growth, and reproductive health. It covers major endocrine glands including the hypothalamus, pituitary, thyroid, adrenals, pancreas, ovaries/testes, and pineal gland, along with their key functions. The module explains hormone classification, receptor signaling, cellular responses, and the differences between peptide, steroid, and amino acid-derived hormones. It also covers negative and positive feedback mechanisms, hormonal axes such as HPA, HPT, and HPG, and the interconnected nature of endocrine function. Special emphasis is placed on neuroendocrine integration and the relationship between hormones, the nervous system, stress, sleep, nutrition, and neurotransmitters.