Clumped isotope geochemistry is concerned with the state of ordering of rare isotopes in natural materials. That is, it examines the extent to which rare isotopes (D, C-13, N-15, O-18, etc.) bond with or near each other rather than with the sea of light isotopes in which they swim. Abundances of isotopic 'Clumps' in natural materials are influenced by a wide variety of factors. In most cases, their concentrations approach (within ca. 1%, relative) the amount expected for a random distribution of isotopes. Deviations from this stochastic distribution result from: enhanced thermodynamic stability of heavy-isotope 'clumps'; slower kinetics of reactions requiring the breakage of bonds between heavy isotopes; the mass dependence of diffusive and then no-gravitational fractionations; mixing between components that differ from one another in bulk isotopic composition; biochemical and photochemical fractionations that may reflect combinations of these simpler physical mechanisms; and, in some cases, other processes we do not yet understand. Although clumped isotope geochemistry is a young field, several seemingly promising applications have already emerged. Most importantly, it appears that proportions of C-13-O-18 bonds in carbonate minerals are sensitive to their growth temperatures, independent of bulk isotopic composition. Thus, 'clumped isotope' analysis of ancient carbonates can be used as a quantitative paleothermometer that requires no assumptions about the delta O-18 of waters from which carbonates grew. This approach has been used to reconstruct marine temperatures across the Phanerozoic (reaching back to the Silurian), terrestrial ground temperatures across the Cenozoic, thermal histories of aqueously altered meteorites, among other applications. Clumped isotope geochemistry is also placing new constraints on the atmospheric budget and stratospheric photochemistry of CO2, and should be capable of placing analogous new constraints on the budgets of other atmospheric gases. Finally, this field could be extended to encompass sulfates, volatile hydrocarbons, organic moieties and other materials. (C) 2007 Elsevier B.V. All rights reserved.