  A moire pattern is formed when two copies of a periodic pattern are overlaid
with a relative twist. We address the electronic structure of a twisted
two-layer graphene system, showing that in its continuum Dirac model the moire
pattern periodicity leads to moire Bloch bands. The two layers become more
strongly coupled and the Dirac velocity crosses zero several times as the twist
angle is reduced. For a discrete set of magic angles the velocity vanishes, the
lowest moire band flattens, and the Dirac-point density-of-states and the
counterflow conductivity are strongly enhanced.
