Stellar models are a fundamental tool to investigate the physical nature of stellar interiors,otherwise hidden to a direct observation. A change in the input physics used to build up a modelproduce different prescriptions for observable quantities. Thus, by comparing models predictions with astronomical data, we can try to constrain fundamental physics. The case of the 12C(α, γ)16O reaction is discussed in some details. Its relevance in various astrophysical contexts is reviewed.