  Accretion disk models have evolved from Bondi flows in the 1950s to Keplerian
disks in the 1970s and finally to advective transonic flows in the 1990s. We
discuss recent progresses in this subject and show that sub-Keplerian flows
play a major role in determining the spectral properties of black holes.
Centrifugal pressure supported enhanced density region outside the black hole
horizon produces hard X-rays and gamma rays by reprocessing intercepted soft
photons emitted by the Keplerian disk terminated farther out from the black
holes. Quasi-periodic oscillations can also be understood from the dynamic or
thermal resonance effects of the enhanced density region.
