  Several physical problems in particle physics, nuclear physics, and
astrophysics require information from non-perturbative QCD to gain a full
understanding. In some cases the most reliable technique for quantitative
results is to carry out large-scale numerical calculations in lattice gauge
theory. As in any numerical technique, there are several sources of
uncertainty. This chapter explains how effective field theories are used to
keep them under control and, then, obtain a sensible error bar. After a short
survey of the numerical technique, we explain why effective field theories are
necessary and useful. Then four important cases are reviewed: Symanzik's
effective field theory of lattice spacing effects; heavy-quark effective theory
as a tool for controlling discretization effects of heavy quarks; chiral
perturbation theory as a tool for reaching the chiral limit; and a general
field theory of hadrons for deriving finite volume corrections.
