In studying large scale graphene structures it is necessary to consider the grain boundaries between the many single-crystal domains. The disruption of the crystallographic structure has consequences for both the electronic and transport properties. Although there has been much interest in this area in recent years, the size of system makes it difficult for ab initio methods to be applied to large structures and tight-binding models have provided some interesting results . The semi-empirical Extended Hückel Theory (EHT) has advantage of being able to take account of charge reordering and to study very large systems. We have already applied this approach to study electrical transport across organic molecules and carbon nanoribbons. In this paper, we report on the results of EHT self-consistent calculations carried out to investigate the effect of grain boundaries on both the electronic structure and the electrical transport.