Abstract: Dislocations are considered crucial linear defects in the synthesis of heteroepitaxial single crystal diamond. Minimizing the dislocation density is a significant challenge for using diamond in electronics. This especially holds for diamond growth on iridium substrates with a large lattice constant difference of 7.1%. We first discuss several aspects of dislocations in heteroepitaxial diamond nucleation and growth, including their generation, types and characterization. Next, methods to reduce dislocation density are summarized, including increasing dislocation reactions (increasing the diamond film thickness and off-axis substrate growth), removing dislocations (conventional epitaxial lateral growth, pendeoepitaxial lateral growth and patterned nucleation growth), and other methods (three-dimensional growth, metal-assisted termination and using a pyramidal substrate). The dislocation density has been reduced to 6×10⁵ cm⁻², based on the use of a micrometric laser-pierced hole array, a method similar to patterned nucleation growth. To further reduce dislocation density and improve crystal quality, proposed ways of controlling the introduction of dislocations (substrate patterning, buffer layer and compliant substrate methods) are highlighted.