  Spherical and axisymmetric accretion onto black holes is discussed. Physical
processes in various families of solutions are explained and their
characteristics are summarized. Recently discovered solutions of axisymmetric
flow provide us with various radiation efficiency and spectrum, which may
successfully model diverse accretion systems. Possible role of preheating is
also speculated. The various families of solutions can be plotted as
trajectories on the plane of (l,e) [or (l,dot m)] where l is the luminosity in
units of the Eddington luminosity, dot m is the similarly defined mass
accretion rate and e is the efficiency defined by e equiv L/dot M c^2 = l/dot
m. We discuss the domains on these planes where solutions are known or expected
to be unstable to either spherical or on-spherical perturbations. Preliminary
analysis indicates that a preheating instability will occur along the polar
direction of the advection dominated flow for e \gsim 10^{-2}.
