We have investigated on the growth of InGaN layers and quantum dots (QDs) on GaN (0001) by metal-organic vapour phase epitaxy. For indium contents above 15% strained QDs form after a certain layer thickness (Stranski Krastanov mode) with typical heights of 2-3 nm. The QD density increases fast during further deposition until saturation above 10(10) cm(-2). Growing thicker layers results in larger but relaxed structures and leave only a small growth window for strained QDs. The data indicate that QDs form when the accumulated strain energy calculated from indium content and layer thickness exceeds a critical value which is independent from the indium content. The same behaviour was found for the critical thickness of relaxation, indicating that relaxation occurs via the relaxation of large QDs with heights above 3 nm. (C) 2013 Elsevier B.V. All rights reserved.