Transmission electron microscopy studies of the core structure and optoelectronic properties of dislocations in GaN films are described. It is shown that the core structure depends sensitively on the growth method and on the presence of dopants and impurities including Si, Mg and 0, with edge, screw and mixed dislocations all becoming open core type under certain conditions. High-resolution electron energy-loss spectroscopy is used to confirm impurity segregation to dislocations. Electron holography and cathodoluminescence studies showing that dislocations possess band gap states and act as non-radiative recombination centres are reviewed, and correlated, tentatively, to impurity segregation. (c) 2006 Springer Science + Business Media, Inc.