A brief review of the authors’ studies on the molecular nature of the beta, alpha (glass) and "liquid-liquid" (11) transitions in flexible-chain polymers using different thermal analysis techniques is presented. DSC, far infrared spectroscopy versus temperature and dynamic mechanical analysis of statically loaded samples have been used. They enabled, for the first time, the common nature and fundamental interrelationship of the above-mentioned transitions in polymers of different structure to be established experimentally. It was found that the moving segment remained practically invariable over a wide temperature range, for temperatures T greater than or equal to T-beta, and was close to the magnitude of the statistical (Kuhn) segment. Simple relationships between transitions parameters and main molecular characteristics of polymers have been found that offer a new approach for the prediction of transition characteristics, including the manifestation of the glass transition in multicomponent and plasticized polymeric systems, on the basis of the molecular parameters.