The form of the quasar luminosity function and its redshift dependence to z~1 has long been established; powerful evolution is required so that by z=1 there is an increase of order 102 – 103 in the space density of the most luminous sources. However it is more difficult to deduce the form of the LF at high redshifts. In this contribution we discuss how a sample of relatively bright radio sources has been used to determine the high-redshift behavior of the radiodoud quasar luminosity function, and the particular advantages of using a radio-selected sample. Our results illustrate how radio-loud quasar samples can be an efficient probe of the high-redshift. Universe.