This work reviews experimental data and models for the thermal conductivity of nanoparticle suspensions and examines the effect of the properties of the two phases on the effective thermal conductivity of the heterogeneous system. A model is presented for the effective thermal conductivity of nanofluids that takes into account the temperature dependence of the thermal conductivities of the individual phases, as well as the size dependence of the thermal conductivity of the dispersed phase. We demonstrate that this model can he used to calculate the thermal conductivity of nanofluids over a wide range of particle sizes, particle volume fractions, and temperatures. The model can also be used to validate experimental thermal conductivity data for nanofluids containing semiconductor or insulator particles and confirm the size dependence of the thermal conductivity of nanoparticle. (C) 2010 American Institute of Chemical Engineers AIChE J, 56: 3243-3256, 2010