The present research aims to reconstruct the phylogeny of Plio-Pleistocene European deer and to use it as a case study of “adaptive radiation” to try to understand some classic palaeobiological questions such as the link between diversification and environmental change, the origins of endemism, size fluctuations within lineages, and the evolution of sexual organs (antlers) in relation to factors such as habitat and body size. Deer are very abundant in the European Pleistocene and their marked species turnover and high rate of evolutionary change make them useful biochronological indicators and a good example of adaptive radiation. However, a clear understanding of their evolution is biased by their confused taxonomy, and the number of identified remains is reduced by the fact that their systematics is mainly based on antler and cranial morphology, not taking teeth and limb bone morphology into account. I will therefore first review the confused taxonomy of the Pleistocene deer, with special regard to the Megacerini (giant deer) and to the Dama-like deer (fallow deer) also assessing relationships with other taxa, by re-examining the original descriptions and re-studying key specimens from Britain and the continent. Then I will determine metric and non-metric morphological variation in the dental and postcranial remains using articulated skeletons from key sites in Western Europe as a guide. A cladistic analysis based on all available non-metric characters will provide a systematic framework for the exploration of palaeobiological questions. Chronological calibration will allow assessment of speciation rates in relation to environmental change. Geographical and chronological variation in size and adaptive form will be interpreted in relation to climate, habitat and guild diversity, testing ideas on the correlates of body size, antler complexity, and feeding and locomotor adaptation.