Evolution of high-redshift quasars

The discovery of luminous quasars at z > 6 indicates the existence billion-solar-mass black holes at the end of reionization epoch. These quasars provide the best probes of the early growth of supermassive black holes in the universe and the relation between the formation of early galaxies and black holes. With the advent of the Sloan Digital Sky Survey and other large area surveys, about 1000 quasars have been discovered at z > 4, including >50 at z > 5 and nine at z > 6. I here review the recent observational results studies of the highest redshift quasars, including the evolution of quasar density and luminosity function, the evolution of spectral properties and chemical enrichment history, distribution of black hole masses and properties of quasar environment and host galaxies at high redshift.