Simulation of the large changes in Quaternary climates provides a critical test for global climate models. Evaluation of the model results has opened many new opportunities for interdisciplinary Quaternary research and stimulated (a) the compilation of data sets that are global in scope and (b) the development of increasingly sophisticated methods for paleoenvironmental analysis. These data sets and analyses are being used to check the performance of the continually improving climate models. Their expansion into earth system models, which simulate hydrological variations and biome distributions, is making the evaluations with paleohydrological and paleoecological data just that much more straightforward. By showing how the models perform under conditions drastically different from today, the evaluations have provided critical tests for the models and have indicated model components that need improvement. The model results, in turn, have increased understanding of the causes of Quaternary environments and environmental change and shown features and linkages within the climate system that are difficult to infer directly from the data.