The "alphas-2019: Workshop on precision measurements of the strong coupling constant" conference, to be held at ECT* (Trento), aims at exploring in depth the current status and upcoming prospects in the determination of the QCD coupling constant $\alpha_S(m_Z)$ from the key observables where high-precision experimental measurements and theoretical calculations are (or will be) available: (i) lattice QCD, (ii) hadronic decays of tau leptons, (iii) deep-inelastic electron-proton scattering and global parton density analyses, (iv) QCD corrections to electroweak precision observables, and analysis of (v) hadronic final states in high energy particle collisions ($e^+e^-$, ep, and pp).