It has been estimated that the world's oceans contain at least 10 TW of renewable energy potential, comprising wave & tidal energy, ocean thermal energy conversion (OTEC), ocean current, and salinity gradient energy. However, these resources remain largely untapped - currently there is around 500 MW (0.005%) of installed ocean renewable energy capacity throughout the world, and this largely comprises of tidal range energy. This volume of Comprehensive Renewable Energy discusses various aspects of the ocean renewable energy resource and the technologies suitable for ocean energy conversion, covering wave & tidal energy, ocean currents, OTEC, and salinity gradient energy. In this introduction to the volume, I briefly introduce the nature and potential of these five main ocean renewable energy resources, with further details found in subsequent chapters of this Volume.