This book presents current research related to the synthesis, characterisation, and heat transfer of nanofluids. Nanofluids are stable colloidal suspensions of solid nanomaterials in base fluids. While nanoparticles were first added to base fluids to obtain nanofluids; other nanomaterials, like nanorods, nanotubes, nanowires, nanofibers, nanosheets, or other nanocomposites, are used to synthesise the nanofluids. The types of base fluids cover a wide range of liquids that include water, oil, ethylene-glycol (automotive antifreeze), refrigerants, polymer solutions, or even bio-fluids. The special properties of nanomaterials and their interactions with base fluids lead to substantially different properties of nanofluids compared with that of base fluids. Significant physical insights into complex physical phenomena in nanofluids are gained via the utilisation of advanced theoretical tools and state-of-the-art experimental measurement techniques.