Coupling constants are fundamental quantities determining the strength of a force. I will discuss $\alpha_s$, the coupling constant of Quantum Chromodynamics (QCD), the theory of strong interactions. In spite of their name, coupling constants evolve with the distance scale at which the interactions occur. While $\alpha_s$ is well understood at short distance, typically $10^{-16}$ meter, its behavior is less clear at larger distance. After discussing $\alpha_s$ at short distance, I will describe experimental and theoretical works investigating its behavior at large distance. One theoretical framework that will be discussed is Holographic QCD, or AdS/QCD, a semi-classical approximation of QCD. It leads to an excellent parameter-free description of both $\alpha_s$ at large distance and of the hadron mass spectrum. The determination of the hadron mass spectrum has been a long-sought goal of QCD. That this can be achieved with a well-justified parameter-free approximation of QCD is an exciting development.