Recent thermal transport measurements in various materialsincluding pseudogap cuprates have observed large thermal Hall effects which are attributed to phonons. In this talk, I will review some recent experiments and propose a theory of phonon thermal Hall effect based on resonant scattering of phonons off dynamical defects. Using a microscopic formulation based on the Kubo formula, we find that the leading contribution perturbative in the phonon-defect coupling is proportional to the phonon lifetime. This contribution is at resonance when the phonon energy equals a defect level spacing. Our results are obtained for different defect models, and include a model of an impurity quantum spin in the presence of quasi-static magnetic order with an isotropic Zeeman coupling to the applied field, which we propose applies to the cuprate pseudogap regime.