Abstract Stable colloidal suspension of nanoparticles in various solvents is conventionally termed as “Nanofluids.” In this review the various facets of nanofluids technology are described with an emphasis on applications in energy systems. Various synthesis protocols and materials characterization techniques for nanofluids are explored (including non-Newtonian rheological behavior of nanofluids). The root of the controversies in the literature relating to anomalous enhancement in the materials properties is also discussed. The review is focused on energy applications of nanofluids, such as nuclear energy, geothermal energy, solar energy, batteries, and fuel cells as well as oil and gas (exploration and production). Niche applications of nanofluids for immediate commercial deployment were identified as additives for deep drilling fluids in geothermal and oil production (as heat transfer fluids, lubricant and formation of nanocomposites with drilling mud to obviate bore hole issues) and as thermal energy storage media in molten salt nanofluids (for solar thermal applications). In the longer time horizon, nanofluids show promise for thermal management of fuel cells and as novel electrolytes in improving the energy storage in batteries. Reliability is an important issue in nuclear applications and the stability of conventional nanofluids needs to be improved in order to achieve significant penetration in nuclear applications. Nanofluids are also expected to spawn novel energy applications—such as for thermal management of electrical rechargeable batteries, fuel cells, solar thermoelectric platforms for power generation, and novel strategies for energy storage in solar ponds by exploiting the non-Newtonian rheology of nanofluids at high nanoparticle concentration.