The physical properties of thin films affect the performance and durability of nearly every solar energy conversion device. Familiar examples of thin films for solar applications are optical materials and protective coatings. Optimized optical properties are key to cost-effective photothermal conversion where individual components must have high absorptance, reflectance, or transmittance. The protection of sensitive substrates from corrosion and/or erosion is essential to ensure adequate component and system lifetime. Such substrates range from photovoltaic materials operating near room temperature to turbine blade structural alloys in hostile environments at very high temperatures (>1,000 degrees C). Although much has been written on particular categories of thin-film materials for solar energy (for example, absorbers for receiver surfaces), to date no one has provided an overview of the spectrum of applications for passive thin films in solar energy. This work is such an overview and also reviews the material state of the art as described in the current literature. Active thin film devices such as photovoltaics and thermoeleetrics are not discussed.