Using a newly developed in situ curvature measurement technique with nearly perpendicular incidence, we observed the strain state of group-III-nitride layers during growth by metal organic vapour-phase epitaxy. Different sources of compressive or tensile stress could be identified. The impact of 3D-island coalescence, of epitaxial and thermal mismatch, or of doping could be monitored in detail as well as the influence of different layer schemes on stress evolution. The lattice mismatch induces a high dislocation density. A reduction could be achieved by an in situ masking of the GaN layer with a SiN mask deposited using silane and ammonia in the growth chamber. On this mask, a nanoepitaxial lateral overgrowth can be performed partly blocking dislocations. With theoretical models we describe the evolution of stress during growth. We will show ways to control stresses and strains in GaN heteroepitaxy to achieve crack-free, device-relevant GaN layers on Si. (c) 2004 Elsevier B.V. All rights reserved.