The tensile strain in AlGaN layers on GaN is well established to lead to cracking if a critical thickness is reached, unless measures such as interlayers are applied to prevent their formation. However in devices, such as HFETs such an approach is impractical. Growth of AlGaN-GaN structures was carried out by MOVPE using a standard two stage process for the growth of the GaN on sapphire. The crack structures were examined by optical and atomic force microscopy. Studies on thin AlGaN layers on GaN close to the crack critical thickness show the stress centres from which the cracks propagate are threading dislocations with cracks often initially forming to link together these stress centres if they are in close proximity. These cracks then extend and “lock” into the generally observed 〈21 10〉 direction in more highly strained layers. A macroscopically uniform crack array is observed in these thin AlGaN samples.