Post-AGB circumstellar envelopes undergo drastic changes due to the combined action of powerful stellar winds in the proto-PN phase and intense stellar radiation (UV to X-ray) in the PN phase. These effects produce extreme physical conditions in the neutral gas (high densities, temperatures, and velocities) through shocks and photon dominated regions (PDRs), which are now realized to be key features of PN formation. We propose a systematic study of these extreme regions in a selected sequence of proto-PNe and PNe. Measurements will be made of the principal diagnostic atomic lines of the excited neutral gas in envelopes at different stages of evolution. In specially targeted cases we plan to obtain the spatial distribution of the emission, the kinematics of the gas, and (through observations of the ground vibrational state of molecular hydrogen lines) the physical conditions of the molecular gas. The above project is thought to complement the Central Program which does not include studies of the spatial extension of evolved planetary nebulae or high spectral resolution measurements of the main infrared cooling lines. The proposed observations will supply fundamental data on the highly excited gas at different stages of envelope evolution, and will provide new insights into the extreme physical processes which play an important role in PN formation.