  For a closed oriented 3-manifold Y, we define an absolute grading on the
Heegaard Floer homology groups of Y by homotopy classes of oriented 2-plane
fields. We show that this absolute grading refines the relative one and that it
is compatible with the maps induced by cobordisms. We also prove that if {\xi}
is a contact structure on Y, then the grading of the contact invariant c({\xi})
is the homotopy class of {\xi}.
