The remarkable enhancement in heat transfer capabilities of conventional fluids with the addition of nanosized metallic and non-metallic particles appealed the attention of investigators towards the suspension of hybrid nanocomposites as a substitute of mono particles. Although these fluids manifest captivating thermal characteristics, the drawbacks associated with their application include high frictional effects and pumping power requirements. The major cause of aforementioned problems is the elevated viscosity. The current study summarizes the work of different investigators and discusses the critical factors affecting the viscosity of hybrid nanofluids such as temperature, particle concentration, pH value, particle size and morphology with a concise discussion on the reasons reported in the literature for the viscosity augmentation. Furthermore, the models developed by different investigators have also been discoursed with specified limitations. Comparison between the viscosity of mono and hybrid nanofluid is also presented comprehensively. It is observed that most of the studies considered the effect of particle concentration and temperature that the effect of these factors is more significant. Water-based nanofluids delivered better results in comparison of ethylene glycol-based nanofluids while the oil-based nanofluids preferred in the applications where the pumping power is not more significant. It has been noticed that the fluids containing tube shaped nanoparticles comparatively showed enhanced viscosity than that of spherically shaped nanoparticles. It has also been observed that the studies preferred to develop their own models for the prediction of viscosity rather than to use the existing models and failed to provide a universal correlation.