This proposal aims to provide a thorough understanding of the surface chemistry and physics during atomic layer deposition (ALD) of cobalt and nickel for applications in microelectronics and catalysis. In microelectronics, ALD of high quality, continuous metallic films is required, and preferably only on specific parts of the substrate, so-called area-selective ALD. As for catalysis, deposition of metal or bimetallic nanoparticles with precise control over size, composition and coverage is needed. To achieve this, a detailed understanding of the reaction mechanism, surface chemistry and nucleation mechanism of Co and Ni ALD on various substrates is required. This proposal builds on recently developed chemistries for Co and Ni ALD using metal precursors with diazadienyl ligands. Although the general characteristics and area-selectivity of these processes have been reported, the reaction mechanisms and reasons for the observed selectivity are largely unknown. Therefore we will employ our worldwide unique toolbox of advanced in situ characterization methods to tackle this problem. First, various co-reactants will be explored to understand the surface chemistry during steady state growth. Secondly, the nucleation mechanism will be investigated on various substrates in order to find the reasons for the observed surface selectivity. Thirdly, the nucleation and initial growth will be studied in detail in order to control the final particle/layer morphology.