Quasi-isothermal temperature-modulated differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) measurements (TMDSC and TMDMA, respectively) permit determination of heat capacity and shear modulus as a function of time during crystallization. Nonreversible and reversible phenomena in the crystallization region of polymers can be observed. The combination of TMDSC and TMDMA yields new information about local processes at the surface of polymer crystals, like reversible melting. Reversible melting can be observed in the complex heat capacity and in the amplitude of sheer modulus in response to temperature perturbation. The fraction of material involved in reversible melting, which is established during primary crystallization, remains constant during secondary crystallization for polycaprolactone (PCL) and polyether ether ketone (PEEK). This shows that, also after long crystallization times, the surfaces of the individual crystallites are in equilibrium with the surrounding melt. Simply speaking, polymer crystals are “living crystals.”