Surveys of the Cosmic X-ray Background.

We provide a highly concise overview of what X-ray surveys and their multiwavelength follow-up have revealed about the nature of the cosmic X-ray background (CXRB) and its constituent sources. We first describe early global studies of the CXRB, the development of imaging CXRB surveys, and the resolved CXRB fraction. Second, we detail the sources detected in CXRB surveys describing their identification, classification, and basic nature. Third, since active galactic nuclei (AGNs) are the main contributors to the CXRB, we discuss some key insights about their demographics, physics, and ecology that have come from CXRB surveys. Finally, we highlight future prospects for the field.