Accretion Flows Around Black Holes

Most accreting black holes emit a substantial fraction of their luminosity in hard X-rays/soft $\gamma$-rays of up to $\sim \mathrm{f}\mathrm{e}\mathrm{w}\times 100$ keV. The observations suggest that the radiating gas is very hot, $T>{10}^9$ K, and optically thin. Theoretical attempts to explain the observations are reviewed and unresolved issues are highlighted.