This study provides a critical synthesis on the applications of nanofluids in various types of solar thermal systems. Nanofluids have received attention in recent years due to its importance in various industrial applications especially in the renewable energy field. This work presents the most recent advances of nanofluids in thermal energy storage systems, solar collectors, solar stills, and photovoltaic/thermal systems. With the application of nanomaterials, the efficiency of photovoltaic can increase substantially while reducing the production costs of electricity and manufacturing. This review is structured in three parts: the first part focuses on presenting the latest results for the thermal conductivity, viscosity, specific heat, and the thermal expansion coefficient of nanofluids reported in the literature. The second part of this review is allocated to the applications of nanofluids in different types of solar systems such as solar collectors, solar stills, thermal energy storage systems, and photovoltaic/thermal systems. Finally, the challenges and the bio-engineering safety concerns of using nanofluids in solar energy systems are discussed.