The paper by N. I. Shakura and R. A. Sunyaev discusses the observational appearances of black holes in binary systems. The authors highlight that the outward transfer of angular momentum during matter accretion can lead to the formation of a disk around a black hole, with the disk's structure and radiation spectrum primarily determined by the rate of matter inflow, $\dot{M}$. For $\dot{M} = 10^{-9} \times 10^{-8} M_{\odot} \, \text{yr}^{-1}$, the black hole can be a powerful X-ray source with a luminosity of $10^{37}-10^{38} \, \text{erg s}^{-1}$ and an effective temperature of $1-10 \, \text{keV}$. The optical luminosity of the disk can exceed the solar luminosity when $\dot{M} > 10^{-9} M_{\odot} \, \text{yr}^{-1}$. The re-radiation of X-ray and ultraviolet energy by the outer regions of the disk contributes significantly to the optical radiation spectrum, which is characterized by broad emission lines. Variability in the system can be influenced by the motion of the black hole and the gas flow, as well as possible eclipses. In supercritical accretion regimes, the disk luminosity stabilizes at the critical level $L_{\text{cr}} = 10^{38} (M/M_{\odot}) \, \text{erg s}^{-1}$, and a significant fraction of the accreting matter is ejected at high velocities, transforming the disk's spectrum. This can make the black hole appear as a bright star with a strong outflow. The authors also discuss the observational challenges and potential signatures of black holes in binary systems, emphasizing that they can be hidden among known objects if the system is remote and the visible companion has a weak stellar wind. They conclude that black holes in binary systems can be identified through their X-ray and optical emissions, and the detection of compact X-ray sources with masses greater than $2 \, M_{\odot}$ in binary systems would provide evidence for the existence of black holes in the Galaxy.The paper by N. I. Shakura and R. A. Sunyaev discusses the observational appearances of black holes in binary systems. The authors highlight that the outward transfer of angular momentum during matter accretion can lead to the formation of a disk around a black hole, with the disk's structure and radiation spectrum primarily determined by the rate of matter inflow, $\dot{M}$. For $\dot{M} = 10^{-9} \times 10^{-8} M_{\odot} \, \text{yr}^{-1}$, the black hole can be a powerful X-ray source with a luminosity of $10^{37}-10^{38} \, \text{erg s}^{-1}$ and an effective temperature of $1-10 \, \text{keV}$. The optical luminosity of the disk can exceed the solar luminosity when $\dot{M} > 10^{-9} M_{\odot} \, \text{yr}^{-1}$. The re-radiation of X-ray and ultraviolet energy by the outer regions of the disk contributes significantly to the optical radiation spectrum, which is characterized by broad emission lines. Variability in the system can be influenced by the motion of the black hole and the gas flow, as well as possible eclipses. In supercritical accretion regimes, the disk luminosity stabilizes at the critical level $L_{\text{cr}} = 10^{38} (M/M_{\odot}) \, \text{erg s}^{-1}$, and a significant fraction of the accreting matter is ejected at high velocities, transforming the disk's spectrum. This can make the black hole appear as a bright star with a strong outflow. The authors also discuss the observational challenges and potential signatures of black holes in binary systems, emphasizing that they can be hidden among known objects if the system is remote and the visible companion has a weak stellar wind. They conclude that black holes in binary systems can be identified through their X-ray and optical emissions, and the detection of compact X-ray sources with masses greater than $2 \, M_{\odot}$ in binary systems would provide evidence for the existence of black holes in the Galaxy.