  We determine the lens spaces that arise by integer Dehn surgery along a knot
in the three-sphere. Specifically, if surgery along a knot produces a lens
space, then there exists an equivalent surgery along a Berge knot with the same
knot Floer homology groups. This leads to sharp information about the genus of
such a knot. The arguments rely on tools from Floer homology and lattice
theory, and are primarily combinatorial in nature.
