The hydrogen economy relies on hydrogen as a clean energy carrier that can be produced by water electrolysis. Enormous amounts of water would be required for the storage of a significant part of world’s energy in electrolytic hydrogen. Seawater and the oceans are the most abundant sources of water on Earth and represent 97% of its total water reserves. The concept of seawater electrolysis was proposed in the 1970s and since then, the research was dedicated primarily to the development of direct chlorine-free seawater electrolysis techniques. This chapter provides details on seawater electrolysis chemistry; electrode materials that were specially designed for this process; and describes the seawater electrolysis techniques that have been reported so far in the relevant literature.