A wave entering a harbor will be diffracted and reflected, depending on the shape and the construction of breakwaters, quays and other objects in the harbor. The incoming wave can increase or decrease in height and a resonance can occur. This affects the manouvering of ships and the movement of moored ships, which influences the loading and unloading of cargo. When planning a harbor or an extension of one, harbor authorities will be interested in the wave conditions in the harbor. The phenomena of wave propagation in a constant-depth harbor can be described with diffraction theory. When the depth in the harbour varies, the wave propagation can be described by a combination of refraction and diffraction theory.