This book presents a comprehensive yet accessible overview of the Green-Naghdi (GN) Theory of water waves and its applications in marine hydrodynamics and ocean engineering. It introduces the fundamental ideas behind GN theory, tracing its development and connections to other nonlinear wave theories. The text explains, in clear terms, the theoretical background of GN equations: how they are derived, their various formulations, and how they compare with classical and modern approaches. In turn, the book guides readers through practical applications of GN theory, from wave propagation and shoaling in shallow waters to wave-structure interactions and internal waves in deep waters. It serves as an entry point for students, researchers, and engineers interested in nonlinear wave modelling, offering a concise and coherent path to understanding one of the most versatile frameworks for describing complex water-wave dynamics.