There are a wide variety of solar thermal, photovoltaic, and hybrid photovoltaic–thermal concentrator systems and concepts that use liquids for optical adaptation and heat transfer. Depending on the functional purpose of the fluid: heat transfer, optical adaptation, spectral filtering, or a combination of these applications, different fluid properties are required. This review studies actively cooled solar concentrators and their associated requirements for liquids. The most suitable candidate fluids available in the market are assessed according to their properties and applications, with a special emphasis on fluid toxicity and long-term performance. Copyright © 2012 John Wiley & Sons, Ltd.