Numerical weather prediction has been operational since 1955 (Thompson, 1961; Fawcett, 1962). However, insufficient data and computing power have precluded a similar capability for the ocean despite the potential for numerous military and civilian applications, e.g.: antisubmarine warfare, tactical planning, optimum-track ship routing, search and rescue, long-range weather and climate prediction, sea-ice prediction, fisheries planning, design and protection of underwater structures such as oil figs, and prediction of pollutant dispersion. Obstacles to prediction of ocean circulation arise because, historically, the ocean has been much more difficult to observe. In addition, the spatial scale for meandering ocean currents and eddies (the oceanic “weather”) is an order of magnitude smaller than that for major weather systems and the meandering of the jet stream. It is only now that global-scale, eddy-resolving ocean prediction is at the threshold of feasibility. Four key technical or technological advances have made this possible.