Tides are caused by the gravitational pull of the Moon and to a lesser extent the Sun. Although tides affect all fluid bodies on Earth in some measure, including some parts of the solid Earth itself, their main effect is on the seas and oceans. Ultimately the kinetic energy of tides is converted into heat, mainly through friction between water and the sea bed. Tides can be exploited as an energy resource, and the total amount of power available can be calculated from knowledge of the gravitational effects of the Earth-Moon-Sun system. At about 2.7 TW, it is many orders of magnitude less than the power of solar radiation (Figure 1.7) and less than 20% of the current power demand for human activities.