Over the recent years, addressing solar energy utilization for different applications has grabbed attention of many research groups around the world. From the past few decades, scientists had made progress in innovating new devices and methods for harnessing solar energy. In this respect, they developed new materials to improve energy efficiency as one of the major focal domain. During twentieth century, scientists engineered the application of nanotechnology in various domains including solar thermal conversion devices. Nanofluids, a homogeneous dispersion and stable suspension of nanoparticles in the base fluids, have made possible progress to achieve higher thermal properties at the smallest possible concentrations. This chapter intends to summarize the research done on the nanofluid applications in different solar thermal conversion systems. This chapter includes comprehensive information about thermophysical properties of nanofluids, the design of solar thermal system at optimum conditions, and the applications of solar collector with nanofluid. Also, challenges and opportunities for future research are identified and reported as well. © 2017, Springer International Publishing Switzerland.