The evolution of obscured accretion
  Our current understanding of the evolution of obscured accretion onto
supermassive black holes is reviewed. We consider the literature results on the
relation between the fraction of moderately obscured, Compton-thin AGN and
redshift, and discuss the biases which possibly affect the various
measurements. Then, we discuss a number of methods - from ultradeep X-ray
observations to the detection of high-ionization optical emission lines - to
select the population of the most heavily obscured, Compton-thick AGN, whose
cosmological evolution is basically unknown. The space density of heavily
obscured AGN measured through different techniques is discussed and compared
with the predictions by current synthesis models of the X-ray background.
Preliminary results from the first half of the 3 Ms XMM observation of the
Chandra Deep Field South (CDFS) are also presented. The prospects for
population studies of heavily obscured AGN with future planned or proposed
X-ray missions are finally discussed.
