Over the last 100 plus years, solar thermal energy has been used for residential heating applications, industrial process heating, electricity generation, and thermochemical reactions. Because of the vast number of applications, numerous designs have been developed to improve the efficiency of converting incoming solar energy into useful heat and to lower the cost. Conventional solar thermal collectors required a solid surface to absorb and convert incoming solar energy to useful thermal energy. Developments in materials science have enabled a new type of absorber-a volumetric absorber-which utilizes nanoparticles suspended in a fluid to absorb sunlight. Since most working fluids only weakly absorb sunlight, well-engineered 'nanofluids' are attractive because only a low volume fraction of nanoparticles is needed to obtain a large shift in the optical properties. This review, on the 10-year anniversary of the first appearance of nanofluid-based direct absorption solar thermal collectors, provides a forward-looking perspective on the challenges and opportunities associated with nanofluids as direct absorbers. Through a critical comparison of design considerations, as well as the most recent experimental results of less well explored areas like hybrid photovoltaic/thermal systems and direct steam generation, this review aims to provide discourse on the next steps for development. (C) 2019 Elsevier Ltd. All rights reserved.