Nanofluids are a new class of fluids engineered by dispersing nanometer-size structures (particles, fibers, tubes, droplets) in base fluids. In the last decade though the researcher has focused on the development in the field of nanofluids science with the emphasis on heat transfer but attention should also be keen on nanofluids preparation since the final properties of nanofluids are dependent on the stability of the dispersion. The current chapter focus on the recent progress on the study of nanofluids, such as the preparation methods, stability of nanofluids, methods to enhance the stability for nanofluids, the stability mechanisms of nanofluids, and presents the broad range of current and future applications in various fields including energy and mechanical and biomedical fields. This chapter also identifies the opportunities for future research. In addition to this, the chapter also focuses on a new class of nanofluids termed as â€œIonanofluidsâ€�. Ionanofluids represent a new and innovative class of heat transfer fluids that encompass multiple disciplines like nanoscience, mechanical, and chemical engineering. This ionanofluids exhibit enhanced thermal properties compared to their base ionic liquids (ILs) which further increase with increasing concentration of carbon nanotubes (CNTs) as well as fluid temperature to some extent. This chapter concludes that ionanofluids (INFs) show great promises to be used as innovative heat transfer fluids and novel media for the exploitation of green energy technology.