Strain-induced crystallization (SIC) in natural rubber (NR) has been extensively studied. In this presentation, some aspects of the involved physical mechanisms, which are fundamental for understanding SIC at equilibrium and under dynamic conditions will be discussed. Particular emphasis will be put on the strain relaxation effect that accompanies SIC in both static and dynamic conditions. This fundamental effect is described in the seminal theory of SIC developed by Flory, which we present from an innovative perspective to emphasize its deep analogy to the liquid-gas phase transformation. Even though Flory, theory has only been qualitatively verified experimentally and is limited to static and equilibrium conditions, it grasps the essential of the driving mechanisms at play. Some simple experiments are presented within this framework that should enlighten the most fundamental aspects of SIC. Considering, for instance, the hardening sequence observed beyond SIC during stretching, it is clear that no prediction of the stress level can be made without knowing both the crystalline content and the average elongation of the remaining molten chains in the amorphous fraction. In practice, crystallization kinetics underlies most aspects of SIC. Understanding crystallization kinetics is essential for explaining the hysteretic behavior observed in dynamic conditions. Similarities and/or differences between dynamic and static SIC will be discussed.