Decreases in the concentration of ozone in the stratosphere are predicted to result in more biologically damaging ultraviolet-B radiation (UVBR) reaching the Earth's surface in the near future. The consequences for plant life of this small change in the solar spectrum are currently uncertain; this is particularly true for aquatic macrophytic algae. The aim of this review is to summarize the current knowledge of UVBR effects on macrophytes, and to highlight areas for future research. An overview of published underwater UVBR measurements shows that penetration of this waveband is low in the coastal waters where most macroalgae are found. Knowledge of UVBR targets in macroalgae is poor, and biological weighting functions have yet to be established. Studies of higher plants and phytoplankton are used to illustrate the many possible targets and effects of UVBR. The role of photosynthetically active radiation in causing dynamic and chronic photoinhibition in macroalgae and the interactions of PAR with UVR are examined in detail. These interactions are central to interpretation of field measurements of photoinhibition under natural solar radiation conditions. Potential strategies and mechanisms for repair of UVBR damage are discussed. Before the long-term effects of UVBR on macroalgal production and community structure can be determined, a greater knowledge of the specific targets of UVBR in macrophytes and the mechanisms of protection and repair must be determined. The ability of macrophytes to acclimate to higher UVBR and the interaction of UVBR with other stresses, such as high PAR, temperature and desiccation, must be assessed. Studies are needed which encompass effects on all stages of the macrophyte life cycle, from recruitment to maturity.