Study and research around heat conduction mechanisms of nanofluids and suspensions are active for a decade. Several models of effective thermal conductivity of nanofluids have been published. This review summarizes recent research for thermal conductivity of a solid/liquid suspension and unfolds the effects of interfacial layers, shells, and aggregate structure on this property. Basically, experimental, and numerical, both, ways to determine thermal conductivity are summarized here in respective sections along with an introductory section named preparation of nanofluids. Key parameters affecting the preparation and stability of nanofluids are reviewed here and further scope of research is assessed. Numerical or theoretical thermal conductivity models are validated against experimental ones by some of the authors, which are reviewed and summarized here. The effect of different parameters pertaining to both nanoparticles and base fluids are reviewed and summarized here for different combination of metal/oxides as particles and water/oils as base fluids. Other properties such as viscosity, convective heat transfer, boiling heat transfer of nanofluids and suspensions are equally important and will be reviewed in subsequent reports.