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We revisit the classical view that quasar contribution to the reionization ofhydrogen is unimportant. When secondary ionization are taken into account, inmany plausible scenarios for the formation and growth of supermassive blackholes quasars contribute substantially or even dominantly at z>8, althoughtheir contribution generally falls below that of star-forming galaxies by z=6.Theoretical models that guide the design of the first generation of redshifted21 cm experiments must, therefore, substantially account for the quasarcontribution in order to be even qualitatively accurate.

Relative Role of Stars and Quasars in Cosmic Reionization