Depletion of conventional resources of energy and continuous elevation in average temperature of earth result in the usage of alternative energy resources, and solar energy is the biggest source in one of them. Different solar thermal collectors have been used to store heat energy from the sun and can be used in various domestic and commercial applications. Solar collector efficiency depends on the heat carrying capacity of the working fluid. Thermal effectiveness of the fluid is the combined effect of thermophysical properties of the fluid, i.e., thermal conductivity, viscosity (pressure loss and pumping power), specific heat capacity (capability of storage), and density. Conventional fluids, i.e., water, ethylene glycol (EG) and heat transfer oils, have been used in solar collectors. However, they have low thermal conductivity and high viscosities. Suspension of small concentration of nanoparticles of 1�100 nm in size in the conventional fluids has proven to display better thermal and optical properties than base fluids. Hence, increase in collector�s efficiency has been reported by many researchers. This chapter aims to provide information on the calculation of concentration of nanoparticles and base fluids, preparation of nanofluids, measurements of thermal and rheological properties, and the application in different solar collectors. The major part of the chapter is allocated to the effects of nanoparticles to the base fluid�s properties. Prediction models of different properties of nanofluids and correlation between the properties are also discussed in this chapter. © 2021, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.