  We present a simple and powerful method for extrapolating finite-volume Monte
Carlo data to infinite volume, based on finite-size-scaling theory. We discuss
carefully its systematic and statistical errors, and we illustrate it using
three examples: the two-dimensional three-state Potts antiferromagnet on the
square lattice, and the two-dimensional $O(3)$ and $O(\infty)$ $\sigma$-models.
In favorable cases it is possible to obtain reliable extrapolations (errors of
a few percent) even when the correlation length is 1000 times larger than the
lattice.
