  Using a radiation-gasdynamics code the evolution of spherical planetary
nebulae is followed, while taking into account the evolution of central star
and the fast wind. These models show the importance of ionization fronts for
the structure of planetary nebulae, especially for the so called multiple shell
nebulae (MSPNe). It is shown that the outer shell is formed by the ionization
front while the inner shell is swept-up by the fast wind. These models explain
the emission profiles of the outer shells as well as their various kinematic
properties. Because they are shaped by the ionization front these outer shells
only give indirect information on the AGB mass loss history. The models
indicate that typical MSPN structures point to mass loss variations during the
AGB phase. The ionization also leads to a stalling of the expansion of the
nebula, leading to nebulae with expansion ages lower than their evolutionary
age. Values for ionized mass and Zanstra temperatures are derived from the
models.
