Due to the lack of commercially available large size native substrates, reduction of threading dislocations remains one of the main challenges of group III-nitride based technology. This paper reviews major methods aimed at producing high-quality heteroepitaxial nitride films with reduced densities of dislocations. Methods such as, incorporation of buffer layers, substrate nitridation, interlayer insertion, silaneammonia treatment, lateral overgrowth and growth of thick nitride layers are reviewed. Advantages and drawbacks of these methods are discussed and compared.Introduction One of the main problems of the group III nitride-based technology is the lack of commercially available large-size native substrates. Sapphire or 6H-SiC substrates, are typically chosen for most nitride-based applications. These substrates however, are poorly matched to nitride films. There is, for example, a lattice mismatch of 16 and 13 % between c-plane sapphire substrates and GaN and AlN layers, respectively. As a result, the growth of nitride films directly on such substrates leads to the formation of three-dimensional (3D) islands [1,2]. In the initial stage of the growth, these slightly twisted/tilted islands coalesce leading to the formation of threading dislocations. Also, threading dislocations are formed to reduce the biaxial compressive strains [3,4] resulting from the lattice mismatch during the growth, as well as from the mismatch in thermal expansion coefficient between the layer and the substrate during the post-growth cooling. As a consequence high densities of threading dislocations, usually in the range of 10