Methods for the determination of the optical properties of both conventional and hybrid nanofluids are presented. The optical properties considered included the extinction coefficient, absorptance, transmittance, and reflectivity. Experimental, theoretical, and numerical approaches for the evaluation of the optical properties are reviewed. The effects of nanoparticle concentration, size, and shape on the optical properties are discussed. The influence of the base fluid on the optical properties is examined. The dependence of the optical properties on the wavelength is investigated. Results of various analytical and numerical procedures and their validation with the experimental measurements are presented. It is concluded that the optical behavior of nanofluids can be optimized by simultaneous consideration of proper size, shape, concentration, nanoparticles material, base fluid, weather condition, and the wavelength range. Hybridization is found to be a promising choice for improving the optical properties of nanofluids. Hybrid nanofluids received an increasing attention recently and there is need for further research in the direction of hybrid nanofluids and their optical properties. As water is commonly used as the base fluid extensive literature is found for water base nanofluids. On the other hand, non-water-based nanofluids with enhanced optical properties are still potential future research areas.