A general catalog of active galactic nuclei (AGN) has been created from identified optical sources of X-ray data from the Hamburg-ROSAT Catalog (HRC) and the Byurakan-Hamburg ROSAT Catalog (BHRC). Optical and multiwavelength data from both catalogs have been examined in order to eliminate erroneous AGN and add new ones. The result is a new catalog of 4253 AGN with X-ray emission (ROSAT BSC/FSC). All of these sources are either AGN or have been classified as candidates. All the multiwave data (gamma, X-ray, UV, optical, IR, radio) have been retrieved for these objects using new catalogs and statistical studies have been made. Attempts have been made to find the relation between the radiative fluxes in different ranges for various types of sources and to determine their characteristics, thereby confirming their membership as AGN. The X-ray properties of these sources have been analyzed in order to find the boundary between normal galaxies and X-ray AGN, and an attempt has been made to find hidden AGN.