  Given a contact structure on a closed, oriented three-manifold $Y$, we
describe an invariant which takes values in the three-manifold's Floer homology
$\HFa$. This invariant vanishes for overtwisted contact structures and is
non-zero for Stein fillable ones. The construction uses of Giroux's
interpretation of contact structures in terms of open book decompositions, and
the knot Floer homologies introduced in math.GT/0209056.
