  Dark energy models with a single scalar field cannot cross the equation of
state divide set by a cosmological constant. More general models that allow
crossing require additional degrees of freedom to ensure gravitational
stability. We show that a parameterized post-Friedmann description of cosmic
acceleration provides a simple but accurate description of multiple scalar
field crossing models. Moreover the prescription provides a well controlled
approximation for a wide range of "smooth" dark energy models. It conserves
energy and momentum and is exact in the metric evolution on scales well above
and below the transition scale to relative smoothness. Standard linear
perturbation tools have been altered to include this description and made
publicly available for studies of the dark energy involving cosmological
structure out to the horizon scale.
