The ocean circulation is driven by wind-forcing and by density differences,  the latter arising through gradients in temperature and salinity. After a brief description  of the ocean  current systems which are presently observed, this course focuses on understanding the physical processes that  determine the spatial pattern and amplitude  of the  currents and their  variability. After a recapitulation of basic principles of geophysical fluid dynamics,  the theory of the steady homogeneous wind-driven ocean circulation will be  presented. It leads to an explanation of the presence of  strong  western boundary currents in midlatitude ocean basins (i.e.,  the Gulf Stream in the Atlantic Ocean). Subsequently, the midlatitude theory is  extended to include transient phenomena (waves and instabilities) and the effects of stratification. Next, the  problem of the existence of the ocean's peculiar vertical density distribution serves as an introduction to the theory of the planetary density driven (or thermohaline)  circulation. In a similar way, the problems of the dynamical existence of  the equatorial  countercurrent and equatorial undercurrent motivates to consider  the theory  of the equatorial ocean circulation. Finally, a basic view of the processes  governing  the Antarctic Circumpolar Current is presented.