The z>4 Quasar Population Observed by Chandra and XMM-Newton
  The current status of our Chandra and XMM-Newton project on high-redshift (z>4) quasars is briefly reviewed. We report the main results obtained in the last few years for the detected quasars, along with a few (~10%) intriguing cases where no detection has been obtained with Chandra snapshot observations. 
C. Vignali (INAF-Osservatorio Astronomico di Bologna; Dipartimento di   Astronomia, Universita' di Bologna, Italy), W.N. Bra...