  We discuss the basic physical model and the relevant processes responsible
for creating and shaping planetary nebulae out of a cool AGB wind envelope. We
show that a hydrodynamical treatment along the upper AGB leads quite naturally
to more realistic starting configurations for planetaries with density slopes
steeper than r^{-2}. Taking into account photoionization and wind interaction
in a realistic manner, the hydrodynamics of post-AGB wind envelopes leads to
density structures and velocity fields in close resemblance to observations of
spherical or elliptical planetary nebulae.
