Anyone who has stood on a ship or a beach will recognize that wave motions are an essential element of the ocean environment. And in its deeps, along its margins and across its basins, the ocean seethes with a wide spectrum of slow-moving interior waves that are invisible to the casual eye of the surface observer. Their influence may ultimately be felt across the globe, on land as well as at sea, for years, decades, or centuries. The winds that buffet a land-locked observer may seem less like waves and more akin to the continuous rush of water through a pipe or down a river channel. However, viewed from a broader perspective, even the ever-changing weather patterns of the Earth's atmosphere have their own particular wave dynamics.