  Analysis of spectral absorption features has led to the identification of
several distinct outflow components in AGNs. The outflowing gas is evidently
photoionized by the nuclear continuum source and originates in the accretion
flow toward the central black hole. The most likely driving mechanisms are
continuum and line radiation pressure and magnetic stresses. The theoretical
modeling of these outflows involves such issues as: (1) Which of the above
mechanisms actually contributes in each case? (2) How is the gas uplifted from
the underlying accretion disk? (3) How can the intense central continuum
radiation be shielded to allow efficient radiative driving? (4) Is the outflow
continuous or clumpy, and, if clumpy, what is the nature and dynamical state of
the ``clouds''? This review summarizes recent theoretical and observational
results that bear on these questions and outlines prospects for further
progress.
