Hadron resonances decaying under the strong interaction are a challenge for calculations on a Euclidean space-time lattice. I will demonstrate the determination of scattering amplitudes through Luescher's finite-volume method on the simple example of the $\rho$ resonance. The results obtained for the scattering phase shift and the timelike pion form factor aid the determination of the hadronic vacuum polarization contribution to the anomalous magnetic moment of the muon. In the second part of the talk I will discuss progress towards applications of the method to resonances in the charmonium spectrum.