Nanofluids, suspensions of nanoparticles in a base liquid, are promising thermal fluids for achieving effective heat transfer enhancement. Nanofluids provide much higher thermal conductivity enhancement with significant increase in viscosity when compared to suspensions of micron-sized particles and eliminate problems of sedimentation, channel clogging, and tube wall erosion, which limited the applications of conventional suspensions for heat transfer enhancement. Since the first studies reporting the high thermal conductivity of nanofluids in 1995 [1], there has been a tremendous amount of experimental and theoretical research to gain insight into the relevant parameters and governing mechanisms of thermal conductivity enhancement and viscosity increase in nanofluids. In order to give an idea about the rate of expansion of this field, the number of publications about nanofluids per year in the past decade is shown in Figure 5.1 [2]. © 2016 by Taylor and Francis Group, LLC.