Diffusion of contaminants in the ocean is a major factor forming fields of both natural and man-mad substances introduced into the water medium. Without proper understanding of the laws of contaminant diffusion in the ocean it is impossible to choose the correct methods of calculating the transport of biogenic elements, dissolved gases and pollutants. Diffusion processes in the ocean are very com­ plicated because of the larger number of factors influencing the distribution of a substance. In this regard, progress in studying the regularities of contami­ nant diffusion in the ocean can only be reached when combining theoretical and experimental methods. Both theoretical and experimental studies on the diffusion of contaminants in the ocean have been devoted much attention in many countries of the world. The results are being published in a larger number of journals and collected works; however, until now there have been no comprehensive publications on the diffu­ sion of contaminants in the ocean. This monograph summarizes the results obtained on the problem by the author and other researchers. The presentation of theoretical results is combined with the data obtained in diffusion experiments with artificial tracers. Also given are practical recommendations on how to compute the behavior of contaminants in various hydrometeorological conditions. Several parts of the monograph are based on the studies I have made in colla­ boration with A.N. Gerentsway, V.1. Zats, G.S. Karabashev, S.S. Muravyev, A.