  We look at the influence of external fields on systems described by generic
free energy functional of the order parameter. The external force may have
arbitrary spatial dependence, and the order parameter coupling may be
nonlinear. The treatment generalizes seemingly disparate works, such as pure
fluids, liquid and polymer mixtures, lipid monolayers, and colloidal
suspensions in electric fields, fluids and nematics in gravity, solutions in an
ultracentrifuge, and liquid mixtures in laser radiation. The phase lines and
thermodynamic behavior are calculated at the mean-field level. We find a
``surface'' critical point that can be shifted to higher or lower temperatures
than the bulk critical point. Below this point, the transition from a ``gas''
phase to a ``liquid'' phase is first-order, while above it, the transition is
second-order. The second-order line is affected by the spatial dependence of
the force, while the first-order line is universal. Moreover, the
susceptibility may diverge at a finite location ${\bf r}$. Several analytical
expressions are given in the limit where a Landau expansion of the free energy
is valid.
