The evolution of stress in diamond layers is investigated both experimentally and theoretically in order to develop a comprehensive view of the formation of residual stress. A compressive stress maximum associated with grain coalescence, a decreasing stress with increasing layer thickness and strong stress inhomogeneities at the level of the grain size are observed by in situ macro- and ex situ micro-Raman spectroscopy in diamond films grown on silicon (001) substrates. For most diamond deposits on silicon, neglecting wafer bending for the calculation of thermal stress turns out to be an inappropriate approximation, but even the exact modelling of the thermal stress by means of plate theory and finite element calculations only explains a minor part of the observed stress. Detailed finite element calculations reveal that the average thermal stress, and the thermal stress distribution, are largely modified by temperature gradients during deposition, and by film morphology. Tensile stresses can form due to temperature gradients and surface roughness relaxes an essential part of the thermal stress. The expected average stresses are calculated for common cases. Stress measurements using micro-Raman spectroscopy confirm these predictions obtained from modelling. The microstructure, in particular coherency strains, surface energy effects and disclinations, can contribute substantially to the observed compressive stress maximum at small layer thickness. During grain coalescence, the formation of disclinations can be energetically more favourable than small angle grain boundaries. The related stress fields are estimated to be of the order of several GPa. The formation of large local compressive stresses during grain coalescence is confirmed by micro-Raman spectroscopy. At small layer thicknesses, the evolution of stress is dominated by the microstructure and morphology, whereas at higher thicknesses the thermal stress, including bending effects and temperature inhomogeneities during deposition, is more important.