This chapter explores the remarkable complexity and plasticity of the human brain, describing how it develops from birth through adulthood. The brain contains trillions of neural connections—with one cubic millimetre storing approximately two thousand terabytes of data—and forms these connections through environmental experiences. The chapter examines key brain structures (brain stem, limbic system, and cerebrum) and their functions, highlighting the frontal lobes as the “CEO” of the brain which don’t fully mature until around age 25. Two critical developmental processes are explained: “experience-expectant” stimulation (universal processes like vision development) and “experience-dependent” stimulation (individualised learning shaped by environment). The chapter also documents aspects of the brain’s neuroplasticity—its ability to reorganise itself—which is particularly powerful during childhood and adolescence but diminishes with age. This understanding of brain development provides crucial context for understanding how environmental factors, including screen use, can shape neural pathways and behaviours during formative years.