The goal of this chapter is to emphasize some fundamental and critical issues on the rheological behaviour of nanofluids. Some important key points are presented and discussed, in particular from an experimental point of view, considering the shear flow behaviour of nanofluids and the evaluation of dynamic viscosity, the viscoelastic nature of nanofluids and the possible time-dependence behaviour. Lastly, the impact of external stimuli such electric and magnetic fields on the rheological behaviour of nanofluids is also highlighted. Finally, some crucial steps in the rheological developments of nanofluids are suggested.