Finite Volume Effects in Weak Hadronic Decays

In this talk we discuss finite-volume computations of two-body hadronic decays below the inelastic threshold (e.g. $K\to\pi\pi$ decays). In particular we show how the relation between finite-volume matrix elements and physical amplitudes, recently derived by Lellouch and L\"uscher, can be extended to all elastic states under the inelastic threshold. We also provide a derivation of the L\"uscher quantization condition directly in quantum field theory.