The formation of structural defects in GaN grown in non-polar directions is reviewed based on transmission electron microscopy (TEM) studies. Stacking faults (SFs) formed on c-planes and also on prismatic planes bounded by partial dislocations, in addition to full dislocations, are major defects in these layers. Since c-planes are arranged perpendicular to the substrate, these defects propagate to the sample surface through the active areas of the devices and become detrimental for device applications. An established method to decrease the defect density is lateral epitaxial overgrowth (LEO) and pende-oepitaxy. The measured density of SFs in the seed areas is similar to 1.3 x 10(6) cm(-1) and in the 'wing' areas similar to 1.2 X 10(4) cm(-1): a decrease of almost of two orders of magnitude. For overgrown samples, two opposite wings grow in opposite polar directions: [0 0 0 1] (Ga-growth polarity) and [0 0 0 (1) under bar] (N-gFowth polarity) confirmed by convergent beam electron diffraction. Ga-polar wings are wider and often have different height than those grown with N-polarity, therefore planarity of these layers and Cracking at the meeting front of two wings often occur. It is shown that two-step growth using MOCVD leads to satisfactory layer planarity. (c) 2008 Published by Elsevier B.V.