Knowledge of immunology continues to grow exponentially, and identification of lymphocyte subsets, cytokines, and other members of the immune system’s “cast of characters” continues to provide new targets for biologic drugs. This chapter summarizes the basic concepts of immunology, which is the biomedical science that studies all aspects of the immune system: its cells, molecules, and functions in health and disease. The complexity and rapid pace of discovery in immunology are exciting and clinically promising, even as they present challenges in staying current in this fast-moving field. The chapter explores the latest concepts of innate and adaptive immunity; details the pathophysiology of hypersensitivity, autoimmunity, and immunodeficiency; and builds the foundation for understanding the pharmacology of biological therapies for immune-related disorders. The waxing and waning of immune function across the life span, and implications for care of both children and older adults, are placed in that context.