Hadron resonances from QCD

I will discuss recent progress in utilizing lattice QCD techniques in the study of the hadron spectrum. State-of-the-art calculations are now able to determine hadron-hadron scattering amplitudes as a function of energy and to resolve resonant enhancements corresponding to excited hadrons. Going beyond now well-studied simple elastic cases like the $\rho$ resonance in $\pi \pi$ scattering, I will show results for coupled-channel meson-meson scattering, and consider situations in which a current external to QCD is present, illustrated with the first calculation of the amplitude for $\gamma \pi \to \pi \pi$.