Black holes in binary systems can form accretion disks, which emit X-rays and other radiation. The structure and radiation spectrum of the disk depend mainly on the rate of matter inflow, denoted as $\dot{M}$. When $\dot{M}$ is in the range $10^{-9}-3\times10^{-8}M_{\odot}/yr$, the disk emits powerful X-rays with energies $h\nu\sim1-10$ keV and luminosity $L\sim10^{37}-10^{38}$ erg s$^{-1}$. As $\dot{M}$ decreases, the disk's effective temperature and luminosity drop. When $\dot{M} > 10^{-9}M_{\odot}/yr$, the optical luminosity of the disk exceeds that of the Sun. The optical radiation is due to re-radiation of X-ray and ultraviolet energy from the disk's inner regions. The optical spectrum includes broad emission lines. Variability is linked to the motion of the black hole and gas in the binary system, and may involve eclipses. In some cases, hard radiation can evaporate gas, reducing accretion. When $\dot{M} \gg 3\times10^{-8}(M/M_{\odot})M_{\odot} yr^{-1}$, the disk's luminosity stabilizes at a critical level $L_{cr} = 10^{38}(M/M_{\odot})$ erg s$^{-1}$. A significant fraction of the accreting matter escapes the black hole's gravitational radius, leading to a bright star with high outflow. Black holes do not emit electromagnetic or gravitational waves, so they are observed through their gravitational influence on nearby stars or gas. In binary systems, black holes can appear as 'black' objects, but matter outflow from the visible component can lead to observable emission. In systems with high outflow rates, the disk's luminosity can exceed that of the visible component, with most of the luminosity in the ultraviolet and optical range. Black holes can be among the optical objects, soft X-ray sources, and harder X-ray sources currently studied. Accretion on black holes in binary systems has distinctive features, but they are not as surprising as often assumed. Black holes can be hidden among known objects. In binary systems, a black hole appears as a 'black' object only if the system is remote with a weak stellar wind. Up to 50% of stars are in binary systems, and many can evolve into black holes. The number of binary systems containing black holes could be very large, up to $10^6-10^8$. In binary systems, the stellar wind from the visible component can lead to matter outflow. The outflow can be enhanced by the Roche limiting surfaceBlack holes in binary systems can form accretion disks, which emit X-rays and other radiation. The structure and radiation spectrum of the disk depend mainly on the rate of matter inflow, denoted as $\dot{M}$. When $\dot{M}$ is in the range $10^{-9}-3\times10^{-8}M_{\odot}/yr$, the disk emits powerful X-rays with energies $h\nu\sim1-10$ keV and luminosity $L\sim10^{37}-10^{38}$ erg s$^{-1}$. As $\dot{M}$ decreases, the disk's effective temperature and luminosity drop. When $\dot{M} > 10^{-9}M_{\odot}/yr$, the optical luminosity of the disk exceeds that of the Sun. The optical radiation is due to re-radiation of X-ray and ultraviolet energy from the disk's inner regions. The optical spectrum includes broad emission lines. Variability is linked to the motion of the black hole and gas in the binary system, and may involve eclipses. In some cases, hard radiation can evaporate gas, reducing accretion. When $\dot{M} \gg 3\times10^{-8}(M/M_{\odot})M_{\odot} yr^{-1}$, the disk's luminosity stabilizes at a critical level $L_{cr} = 10^{38}(M/M_{\odot})$ erg s$^{-1}$. A significant fraction of the accreting matter escapes the black hole's gravitational radius, leading to a bright star with high outflow. Black holes do not emit electromagnetic or gravitational waves, so they are observed through their gravitational influence on nearby stars or gas. In binary systems, black holes can appear as 'black' objects, but matter outflow from the visible component can lead to observable emission. In systems with high outflow rates, the disk's luminosity can exceed that of the visible component, with most of the luminosity in the ultraviolet and optical range. Black holes can be among the optical objects, soft X-ray sources, and harder X-ray sources currently studied. Accretion on black holes in binary systems has distinctive features, but they are not as surprising as often assumed. Black holes can be hidden among known objects. In binary systems, a black hole appears as a 'black' object only if the system is remote with a weak stellar wind. Up to 50% of stars are in binary systems, and many can evolve into black holes. The number of binary systems containing black holes could be very large, up to $10^6-10^8$. In binary systems, the stellar wind from the visible component can lead to matter outflow. The outflow can be enhanced by the Roche limiting surface