In this work, we use the effective medium approximation (EMA) to numerically assess the thermal performance of thermal interface materials (TIMs). The performance is measured in terms of the effective thermal conductivity of the composite material subject to changes in the filler dimensions, volume fraction, aspect ratio, filler-matrix interface resistance and thermal conductivity of carbon nanotubes (CNTs) and spherical metallic fillers embedded in non-electrically conducting matrices. Our results indicate that the filler-matrix interface resistance and CNT aspect ratio play a dominant role in limiting the maximum effective thermal conductivity of the composite material. Guidelines for the formulation of these materials are presented as result of the analysis. © 2010 EDA Publishing/THERMINIC.