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The late Miocene (11.6-5.3Ma) represents a potential analogue for future climate. Qualitatively, it was warmer and wetter than today but CO2 reconstructions suggest atmospheric concentrations were not dissimilar to the present concentration of 400ppm. Study of late Miocene climate could therefore inform potential future climate, if our current climate reaches equilibrium with present CO2. For this, however, it is essential to separate out the contributions to climate of palaeogeography change and C02 forcing. This is achieved in this thesis by using an extensive database of late Miocene climate reconstructions for the terrestrial and marine realms in a set of comprehensive quantitative model-data comparisons. These comparisons have been conducted in a manner that incorporates many of the uncertainties in the model and the data such that results obtained are robust. This research finds that even incorporating large uncertainties, there is confidence that aspects of late Miocene climate were markedly different to today. :vIean annual air temperatures were at least 2.5°C warmer than today in Europe and 5°C warmer than today in East Asia and sea surface temperatures were I-2°C warmer than today in the Tropics. These differences cannot be reconciled with late Miocene climate model simulations within the range of palaeo-C02 uncertainty for the late Miocene of lS0-400ppm. A closer examination of palaeotemperature reconstructions from foraminifera finds that uncertainties in seasonality and production depth cannot explain the uniformly cold temperatures reconstructed and the apparent discrepancies with other marine temperature reconstruction techniques. It is not therefore possible to rule out diagenetic alteration of these data. Palaeogeography plays an important role in the determination of late Miocene climate, and therefore aspects of the climatology will not be a suitable analogue for future climate because palaeogeography and C02 forcing do not add linearly. Vegetation feedbacks are also found to be very important in the determination of late Miocene climate and these do not add linearly to contributions from C02 forcing. Late Miocene climate is O.5°C more sensit~ve to CO2 doubling than we might expect future climate to be because of the greater land mass at high northern latitudes and the different vegetation distribution that results.