Atomic layer deposition (ALD) is the method of choice for the deposition of ultrathin films with a high conformality and with precise thickness control. The extension of the technique with plasma processes (i.e., plasma-assisted ALD) provides several potential advantages over thermal ALD such as an enhanced growth rate, improved material properties, and lower deposition temperature. However mechanistic studies are scarce and detailed insight into the surface processes of plasma-assisted ALD is still lacking while being imperative for further process optimization. In this contribution surface processes during plasma-assisted ALD are addressed on the basis of studies of oxide (Al@sub 2@O@sub 3@, HfO@sub 2@) and metal nitride (TiN, TaN) film growth employing several in situ diagnostics. From in situ spectroscopic ellipsometry and quartz crystal microbalance measurements precursor adsorption reactions are discussed addressing the formation of surface species, substrate-inhibited growth, and film nucleation. The surface reaction products as well as the consumption of precursor and plasma species are discussed from time-resolved mass spectrometry studies as well as from optical emission spectroscopy, a unique feature provided by the plasma process. On the basis of the results, the paradigms for plasma-assisted ALD are reviewed and differences with thermal ALD processes are discussed.