We analyzed the role of aggregation and interfacial thermal resistance on the effective thermal conductivity of nanofluids and nanocomposites. We found that the thermal conductivity of nanofluids and nanocomposites can be significantly enhanced by the aggregation of nanoparticles into clusters. The value of the thermal conductivity enhancement is determined by the cluster morphology, filler conductivity and interfacial thermal resistance. We also compared thermal conductivity enhancement due to aggregation with that associated with high-aspect ratio fillers, including fibers and plates. (C) 2007 Elsevier Ltd. All rights reserved.