Molecular line data for the circumstellar envelopes of AGB red giant stars is analyzed to identify the mass-loss mechanisms and any relationships between mass loss and succeeding planetary nebulae. Mass-loss rates and outflow velocities are derived and, when compared with the stellar luminosities and optical depths of the dust envelopes, indicate that radiation pressure drives outflowing dust to its terminal velocity. Departures from this model, observed in OH 2312.8 + 4.2 and planetary and protoplanetary nebulae, had luminosities which declined after planetary nebula formation commenced. 58 references.