This paper provides a review of the technology associated with Positron Annihilation Lifetime Spectroscopy (PALS) as applied to polymers. It is not a comprehensive review of the many technical papers published on PALS. Instead it attempts to consolidate the strong theoretical principles upon which the PALS is based, apd to correlate those principles with the equally strong theoretical principles associated with the physical aging of polymers. Both sets of principles are centered around the concept of free volume, and there is a strong, useful relationship between the molecular scale measurement technique of PALS and the response of a polymer to physical aging. In essence, the free Volume measured by PALS is the same free volume that is affected by physical aging. This relationship was demonstrated with data, obtained from a variety of sources, on bisphenol A polycarbonate, a polymer in which free volume-mechanical property behavior is readily demonstrated.