  We investigate the low-energy electronic transport across grain boundaries in
graphene ribbons and infinite flakes. Using the recursive Green's function
method, we calculate the electronic transmission across different types of
grain boundaries in graphene ribbons. We show results for the charge density
distribution and the current flow along the ribbon. We study linear defects at
various angles with the ribbon direction, as well as overlaps of two monolayer
ribbon domains forming a bilayer region. For a class of extended defect lines
with periodicity 3, an analytic approach is developed to study transport in
infinite flakes. This class of extended grain boundaries is particularly
interesting, since the $K$ and $K'$ Dirac points are superposed.
