There is currently a wide range of technologies for harnessing the energy potential of the world’s seas and oceans. These include a few large barrages built in certain coastal waters, various smaller types of turbine further out to sea capturing wave power, and tidal lagoons where large walls are constructed to trap water at high tide and then release it through turbines at low tide. The challenge of making these technologies work is huge, but so is the prize. Once in operation, they produce no greenhouse gases, and given what we know about the severity of climate change, this is of colossal importance. Any new technology will inevitably affect the environment to some degree. However, the evidence suggests that barrages and underwater turbines have a relatively benign effect. There are also bound to be people who think barrages are an eyesore. These structures, however, are often not easily visible from land and, compared with nuclear power reactors and wind farms, are inoffensive. Suitable locations for capturing tidal energy include those with large differences in tidal range, which is the difference between high tide and low tides, and where tidal channels and waterways become smaller and tidal currents become stronger. Because water is denser than air, tidal energy is more powerful than wind energy, producing exponentially more power at the same turbine diameter and rotor speed.