2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/10819We review current theoretical understanding of the spectral properties (low and high states, transition of states, quasi-periodic oscillations etc.) of the low mass as well as supermassive black hole candidates.5 latex pages including figures. jkas.sty and psfig.tex included. Invited review lecture. To appear in the Proceedings of 6th Asia-Pacific Conference (Journal of Korean Astronomical Society) August, 1996, Eds. H.M. Lee and S. KimAstrophysicstext