  The low-energy theory of interacting electrons on graphene's two-dimensional
honeycomb lattice is derived and discussed. In particular, the Hubbard model in
the large-N limit is shown to have a semi-metal - antiferromagnetic insulator
quantum critical point in the universality class of the Gross-Neveu model. The
same equivalence is conjectured to hold in the physical case N=2, and its
consequences for various physical quantities are examined. The effects of the
long-range Coulomb interaction and of the magnetic field are discussed.
