1 Introduction

1.3 Structure of this review, other relevant reviews, and definitions

2 The main cosmological X-ray surveys, their supporting observations, and AGN selection

2.3 AGN selection from the general X-ray source population

3 AGN demographics

3.2 X-ray luminosity functions and the luminosity dependence of AGN evolution

3.4 The Soltan argument for X-ray selected AGNs

3.5 The environmental dependence of AGN evolution

4 AGN physics

4.1 AGN X-ray obscuration: Basic properties, luminosity dependence, and redshift dependence

4.2 Basic X-ray-to-optical/UV spectral energy distribution properties

4.3 X-ray continuum shape as an estimator of Eddington ratio

5 AGN ecology

5.2 Host-galaxy masses and colors

5.3 Host-galaxy morphologies

5.4 AGN fraction and Eddington-ratio distribution

5.5 Star formation and specific star formation rates

6 Future prospects

6.1 Remaining big questions

6.2 Additional targeted X-ray surveys with operating missions

6.3 Multi-wavelength follow-up observations of X-ray surveys

6.4 New X-ray survey missions