Atomic layer deposition (ALD) is an ideal technique for depositing amorphous and polycrystalline films that require atomic layer control over the film thickness and chemical composition, and can provide conformal coverage with sharp interfaces on high-aspect-ratio nanostructures. ALD has been used to deposit oxide, nitride, elemental and compound semiconductors, and metal films for various applications. The primary challenges in ALD are control over the film’s microstructure and residual impurities, which in turn affect their structural, electrical, and transport properties. To overcome these problems, an understanding of atomic-scale events that occur during growth is required. In this presentation, we will discuss the mechanisms of the surface reactions of radicals and molecular precursors supplied during growth using a suite of in situ diagnostic tools. We will specifically discuss the reaction mechanisms during plasma- and ozone-assisted ALD of TiO2, Al2O3, and SiO2, and how such reactions influence the film properties.