Lattice gauge theory (LGT) is a first principles method for calculating the non-perturbative effects of the strong interaction. The action is discretized on a lattice of points in space-time and physical quantities are computed numerically using the path integral. This approach has been useful in a variety of areas, but I will focus on the light quark sector where LGT can be used to interpret precision measurements of masses and decay processes. This allows us to test the important theoretical constraint of unitarity in the CKM matrix, thus probing the possibility of new flavour physics beyond the Standard Model. I will present results from work on light pseudoscalar masses and decay constants, as well as work on kaon to two pion decays.