  Maximally supersymmetric gauge theory in four dimensions admits local
boundary conditions which preserve half of the bulk supersymmetries. The
S-duality of the bulk gauge theory can be extended in a natural fashion to act
on such half-BPS boundary conditions. The purpose of this note is to explain
the role these boundary conditions can play in the Geometric Langlands program.
In particular, we describe how to obtain pairs of Geometric Langland dual
objects from S-dual pairs of half-BPS boundary conditions.
