Increased UV-B radiation (280 - 315 nm) reaching the surface of the sea due to decreased ozone layer thickness stimulated research on the effects of this most deleterious part of solar radiation on phytoplankton. Among these effects are damage of the photosynthetic apparatus, DNA and other cellular targets. Since UV-B in the solar spectrum has been present always during evolution, phytoplankton have developed mechanisms for avoidance as well as adaptation. These include active positioning in the water column, repair of UV- induced DNA damage and the synthesis of UV- absorbing compounds such as mycosporine-like amino acids (MAAs). In many cases these mechanisms seem not to be directly induced by UV-B radiation rather than by UV-A (315 - 400 nm) or PAR (400 - 700 nm). Susceptibility to UV-induced damage varies greatly between phytoplankton species and also depends on a number of physiological and physical determinants such as nutrient supply and water column mixing. Recent results on this complex matter are summarized and discussed in this review.