Although the approximate position and strength of the main ocean currents have been known for upwards of two hundred years, it is only in the last few decades that oceanographers have attempted to explain them on the basis of sound physical theory. For many years the progress of oceanography was hampered by the lack of sufficient experimental data; today much new information is being gathered throughout the world, enabling existing theories to be examined critically and providing a basis for new work. It appears from the latest investigations that ocean currents are subject to quite large ‘synoptic’ changes superimposed on the general ‘climatic’ pattern in a similar manner to the movement of air masses in the atmosphere, but on a much reduced scale. This and other developments of theory during the century will be discussed in this paper.