Introduction After more than 20 years' research into the effect on plants of elevated levels of UV-B radiation, one might expect it to be a relatively easy matter to predict the impact of future changes in UV-B climate on the growth and yield of important field crops, but this is not the case. Predicting future changes in ground-level UV-B is itself proving very difficult because of uncertainty in trends for future emissions of ozone-destroying chemicals, incomplete knowledge of the atmospheric chemistry involved (see chapters by Webb and by Pyle, this volume) and possible interactions between UV-B and other climate change variables such as CO2 and temperature. Even if future trends were known, recent reviews (for example, Caldwell & Flint, 1994; Fiscus & Booker, 1995) have highlighted the methodological limits of many studies of plant responses to UV-B which make problematic any direct extrapolation to predict likely yield losses at the field-scale. This review of the existing knowledge on field crop responses to UV-B focuses specifically on potential effects on yield. We consider controlled environment, glasshouse and field studies with two questions in mind: (a) will elevated UV-B irradiance alter yields or quality of field-grown crops, and (b) are there beneficial/adaptive effects of UV-B which might be exploited? Unless otherwise stated, the UV-B quantities discussed below are all in terms of the Caldwell generalised plant action spectrum normalised to 300 nm (PAS300; Caldwell, 1971; Caldwell et al., 1986) as discussed by Holmes (this volume).