Advances in technology miniaturization with increasing power density call for new technologies for enhancing heat transfer. Enhancement of heat transfer with the use of nanofluids has been a hectic topic of research and development since the term "nanofluid" was first used in 1995, mainly because the thermophysical properties of nanofluids in most reports in the literature showed supremacy or improvement over their base fluids, which may not allow fulfillment of the present cutting-edge technology needs. Significant progress in this field has been made in the past two decades. This review summarizes a variety of the experimentally measured thermal properties of common nanofluids, the enhancement mechanisms discovered or hypothesised, the models used for properties and heat transfer characteristics, and the applications of nanofluids for enhancing heat transfer. The model of an artificial neutral network is particularly emphasized. Applications to cooling technology, renewable energy and energy systems, and building technology are detailed. Challenges and areas for future research are identified.