Introduction The study of how organisms adapt to temperature has a long and distinguished history. During the past few decades much attention has been directed at understanding the mechanisms by which such adaptation is achieved. At the same time palaeobiologists have been attempting to unravel the influence of climate on the evolution and extinction of the Earth's biota, a concern heightened by the recent realisation that mankind's own activities may already have initiated a significant experiment in the impact of climate change. In this chapter I discuss the impact of past climate change on organisms, using results from palaeobiology, environmental physiology and comparative ecology. A comprehensive review of these fields would require a whole book and so for this chapter I have necessarily been selective, concentrating on those areas that might help generate a coherent picture of how organisms respond to climate change. I have attempted to emphasise both areas of agreement and what appear, with current knowledge, to be significant mismatches between different fields of enquiry. In doing so I have tried to balance citation between key historical literature, comprehensive reviews and important recent work. Space constraints mean that usually only a single illustrative example can be used and I have quite deliberately chosen some of these from the older literature; it is not always the most recent work that is either the best or the most relevant.