  We discuss the observational consequences of black hole spin in X-ray
binaries within the framework of the standard thin accretion disk model. When
compared to theoretical flux distribution from the surface of a thin disk
surrounding a Kerr black hole, the observed X-ray properties of the Galactic
superluminal jet sources, GRO J1655-40 and GRS 1915+105, strongly suggest that
each contains a black hole spinning rapidly in the same direction as the
accretion disk. We show, however, that some other black hole binaries with an
ultra-soft X-ray component probably harbor only non- or slowly-spinning black
holes, and we argue that those with no detectable ultra-soft component above
1-2 keV in their high luminosity state may contain a fast-spinning black hole
but with a retrograde disk. Therefore, all classes of known black hole binaries
are united within one scheme. Furthermore, we explore the possibility that
spectral state transitions in Cyg X-1 are simply due to temporary disk
reversal, which can occur in a wind accretion system.
