In this paper recent work on the application of in situ spectroscopic ellipsometry (SE) during thin film synthesis by atomic layer deposition (ALD) is reviewed. In particular, the versatility of this all-optical diagnostic is demonstrated by results obtained on Al2O3, HfO 2, Er2O3, TiO2, Ta2O 5, TiN and TaNx films with thicknesses ranging from 0.1 to 100 nm. By acquiring SE data in between the ALD cycles and by analysing the film thickness and the energy dispersion of the optical constants of the films, the layer-by-layer growth and material properties of the films can be studied in detail. The growth rate per cycle and the ALD saturation curves can be determined directly by monitoring the film thickness as a function of the number of cycles, while also the nucleation behaviour of the films on various substrates and submonolayer surface changes during the ALD half-cycles can be probed. The energy dispersion relation provides information on the optical properties, the crystalline phase and the material composition of the films. For metallic films, electrical properties can be calculated from the Drude absorption yielding insight into the electrical resistivity and electron scattering effects in ultrathin films. © 2009 IOP Publishing Ltd.