  The Heisenberg spin ladder is studied in the semiclassical limit, via a
mapping to the nonlinear $\sigma$ model. Different treatments are needed if the
inter-chain coupling $K$ is small, intermediate or large. For intermediate
coupling a single nonlinear $\sigma$ model is used for the ladder. Its predicts
a spin gap for all nonzero values of $K$ if the sum $s+\tilde s$ of the spins
of the two chains is an integer, and no gap otherwise. For small $K$, a better
treatment proceeds by coupling two nonlinear sigma models, one for each chain.
For integer $s=\tilde s$, the saddle-point approximation predicts a sharp drop
in the gap as $K$ increases from zero. A Monte-Carlo simulation of a spin 1
ladder is presented which supports the analytical results.
