This chapter explores the emerging role of nanofluids as advanced heat transfer media in Parabolic Solar Trough Collectors (PTCs), offering a promising path to enhance the efficiency of solar thermal systems. Traditional heat transfer fluids, such as water, oils, and molten salts, often suffer from low thermal conductivity, which limits system performance. To address this, researchers have developed nanofluids—engineered by dispersing nanoparticles into conventional fluids—to significantly improve thermal conductivity, heat transfer, and overall system efficiency. The chapter discusses various types of nanoparticles, base fluids, and the thermo-physical properties that influence their performance. It also examines hybrid nanofluids, recent advancements, and the practical challenges involved, such as stability, cost, and compatibility. Concluding the book, this chapter presents a forward-looking view on how nanofluid technology could shape the next generation of high-efficiency solar thermal systems in the global transition to clean energy.