During the last stages of its evolution, low mass stars (<8 Msol) loose about 1 Msol in the form of a cool, low velocity (10 km/s) molecular wind on the Asymptotic Giant Branch (AGB). This phase is thought to end with a burst of increasing mass loss rate (the "superwind") which largely exhausts the star. The star then moves to the left in the HR diagram (the proto- planetary nebula phase, PPN). Once the central star becomes hot enough, the still expanding AGB wind will be ionized. At some point during its evolution, the central star will start losing mass in a fast (100-1000 km/s) but low density wind. This fast wind will drive fast shocks into the AGB wind. The interaction of these winds and the hardening of the UV radiation field will shape the resulting planetary nebula. Understanding this mass loss process and its evolution during the AGB, PPN and PN phases is a key problem within astrophysics because most of interstellar gas and dust originates from these stellar sources. Here, we propose high resolution spectroscopy of far-infrared atomic fine structure lines in fourteen AGB stars, PPNe, and PNe using the ISO LWS and SWS FP spectrometers. Our goal is to determine the physical conditions in the circumstellar envelopes and the relative importance of shocks and far-UV photons in their evolution. The FIR atomic fine structure lines of [CII] 158 um, [OI] 63 and 146 um, [SiII] 35 um, [FeII] 26 um and [SI] 25 um are the primary coolants of warm ( 500K), dense ( 10^6 cm-3) neutral gas and are important probes of photodissociation regions and shock excited regions. The proposed observations will identify and distinguish between warm gas created by shocks and by far-UV photons in PPNe & PNe and will probe the photodissociative effects of the interstellar radiation field on the outer layers of AGB envelopes. The importance of shocks vs. far-UV photons for the warm gas can be directly establish from the observed [CII] (PDR tracer) to [SI] (shock tracer) emission. Furthermore, the SWS FP has a resolution of 10km/s, sufficient to resolve "shocked" line profiles.