During the 20th century, anthropogenic emissions of greenhouse gases have caused global mean temperatures to rise by 0.4-0.8°C. Further warming of 0.4-1.1°C is projected by 2050. Climate change during the 20th century has had effects on many terrestrial and marine ecosystems. This thesis investigates the effects of changes in climate on rocky shore communities in the Bay of Biscay, northeast Atlantic. This is a zone of biogeographical transition with sharp temperature gradients resulting in unusual spatial patterns of cold- and warm-temperate species, likely to be particularly sensitive to climate change. Chapters 2 and 3 collect existing data on climate and species distributions in the region and analyse the data in novel ways. Chapters 3, 4 and 5 present and analyse new data on the distribution of rocky shore species in the region, and on the mechanisms of their responses to environmental gradients in temperature and other physical factors. Chapter 6 develops an original model based on existing and new data on the distribution and responses to climate of key rocky shore species. In Chapter 2 analysis of several data sets showed that both summer and winter, sea and air temperatures in the Bay of Biscay region have on the whole risen since 1950, at a rate consistent with the global average rate of warming. There have, however, been warmer and colder periods, and local variations. Indices of upwelling calculated from alongshore wind stress show increasing trends throughout the 20th century in northwest Spain and northern Portugal; no clear trends were found along the north central coast of Spain. In Chapter 3, analyses of past studies and my surveys during 2000-01 show that the distribution of rocky shore species in the Bay of Biscay has varied considerably since 1895. The abundance and distribution of several common cold-temperate species of brown algae on the north coast of Spain show significant negative correlations with variation in temperature during the 20th century, suggesting that climate change has indeed affected these species. The species studied are ecologically important canopy-forming algae: the changes in distribution observed are thus likely to have had important consequences for rocky shore communities in the region as a whole. In Chapter 4, the responses of two common and ecologically important species of limpet, Patella vulgata L. (cold-temperate) and P. depressa Pennant (warm-temperate), to the gradient in sea temperature along the coast of northern Spain, and the mechanisms governing these responses, were investigated. The abundance of the two species and (especially) two indices of their relative abundance, log₁₀ (Pv/Pd) and Pv(Pv+Pd), were strongly correlated with summer temperature. A similar relationship of relative abundance of the same two species with spatial variations in summer sea temperature was found in data collected in the 1950s in the English Channel.