The present volume is a compilation of knowledge on underwater irradiance as the factor controlling primary organic production in the sea. Together with information gleaned from the literature, it contains mathematical formulas, graphs and tables that are original contributions from the scientists at the Polish Academy of Sciences' Institute of Oceanology to this aspect of oceanology. The model of solar radiation inflow through the atmosphere, water surface and water column down to a depth in the basin is discussed briefly. The spectral characteristics of underwater irradiance and an optical model for classifying water basins by the use of the chlorophyll concentration as an index of the optical type of water are given. The text covers the conditions, characteristics, formulas and tables describing the distributions of irradiance and the light absorption in the water column, as well as the dependence of photosynthesis and its quantum yield on the irradiance. The volume will be found useful in the study and conservation of natural water basins, and for teaching purposes.