  Black holes are by definition black, and therefore cannot be directly
observed by using electromagnetic radiations. Convincing identification of
black holes must necessarily depend on the identification of a very specially
behaving matter and radiation which surround them. A major problem in this
subject of black hole astrophysics is to quantify the behaviour of matter and
radiation close to the horizon. In this review, the subject of black hole
accretion and outflow is systematically developed. It is shown that both the
stationary as well as the non-stationary properties of the observed spectra
could be generally understood by these solutions. It is suggested that the
solutions of radiative hydrodynamic equations may produce clear spectral
signatures of black holes. Other circumstantial evidences of black holes, both
in the galactic centers as well as in binary systems, are also presented.
