Concentrating sunlight is a common practice for increasing the efficiency of solar energy conversion systems, especially solar thermal systems operating at high temperature. Directionally selective surfaces, surfaces which absorb or transmit in certain directions but reflect in others, can also be used to increase the efficiency of such systems. Directional selectivity has been shown to be theoretically capable of achieving efficiencies identical to those under maximum solar concentration. Maximum solar concentration is not achieved in current systems due to practical limitations. This paper investigates the directionally selective properties which would be required to surpass the performance of current concentrating systems.