  We explore a somewhat unexpected connection between knot Floer homology and
shellable posets, via grid diagrams. Given a grid presentation of a knot K
inside S^3, we define a poset which has an associated chain complex whose
homology is the knot Floer homology of K. We then prove that the closed
intervals of this poset are shellable. This allows us to combinatorially
associate a PL flow category to a grid diagram.
