The world’s oceans are a vast source of renewable energy that is typically more predictable than wind and solar power. Promising ocean energy resources that could provide significant amounts of carbon-free power include wave energy, ocean current energy, and ocean thermal energy – and in certain regions, tidal energy as well. Of these technologies, wave and tidal power have made the most progress to date. However, due in large part to the often harsh conditions presented by the ocean, today’s technologies are far from being cost-competitive with other sources of electricity. Meanwhile, ocean thermal energy conversion technology, which extracts energy from temperature differences in the ocean, is still its early stages of development and has not yet demonstrated a path to cost-competiveness, while ocean current energy capture remains largely unexplored. A new generation of transformational ocean energy technologies will need to be developed to allow us to finally unlock the vast carbon-free energy resources that exist in our oceans all around the world.