Numerical Lattice QCD calculations are necessarily performed in a finite volume and with Euclidean time. For scattering, decay and transition amplitudes these constraints severely limit the extraction of physical observables. However, great progress has been made by using finite volume as a tool rather than an artifact, and deriving non-perturbative relations between the finite- and infinite-volume theories. Over a decade ago Lellouch and Luescher derived such relation between finite-volume matrix elements and the $\mathrm{K} \rightarrow \pi \pi$ decay amplitude. In this talk I explore generalizations of this idea to semi-leptonic outstates, with brief discussion on potential applications to B physics.