  In this paper we use one-loop chiral perturbation theory in order to compare
lattice computations of the K+ to pi+ pi0 decay amplitude with the experimental
value. This makes it possible to investigate three systematic effects that
plague lattice computations: quenching, finite-volume effects, and the fact
that lattice computations have been done at unphysical values of the quark
masses and pion external momenta (only this latter effect shows up at tree
level). We apply our results to the most recent lattice computation, and find
that all three effects are substantial. We conclude that one-loop corrections
in chiral perturbation theory help in explaining the discrepancy between
lattice results and the real-world value. We also revisit B_K, which is closely
related to the K+ to pi+ pi0 decay amplitude by chiral symmetry.
