  We study electron transport properties of a monoatomic graphite layer
(graphene) with different types of disorder at half filling. We show that the
transport properties of the system depend strongly on the symmetry of disorder.
We find that the localization is ineffective if the randomness preserves one of
the chiral symmetries of the clean Hamiltonian or does not mix valleys. We
obtain the exact value of minimal conductivity $4e^2/\pi h$ in the case of
chiral disorder. For long-range disorder (decoupled valleys), we derive the
effective field theory. In the case of smooth random potential, it is a
symplectic-class sigma model including a topological term with $\theta = \pi$.
As a consequence, the system is at a quantum critical point with a universal
value of the conductivity of the order of $e^2/h$. When the effective time
reversal symmetry is broken, the symmetry class becomes unitary, and the
conductivity acquires the value characteristic for the quantum Hall transition.
