Ab Initio simulations have become widely used tools in the materials, surface science and chemistry communities for predicting molecular and materials properties and for studying atomistic processes. In this talk we will give an overview of how these methods can be used to understand ALD and MLD from an atomistic viewpoint and what you can learn from simulations. We will also discuss the limits of quantum chemistry and provide guidelines for those interested in using simulations to study ALD. Examples from our past and current work as well as that of others will be used to illustrate the application of these methods to surface reactions relevant to ALD and MLD. Examples include; the effect of using precursors with different ligands, including alkoxides, alkylamides and halogens, to deposit HfO2 and Al2O3; the effect substrate type and functionalization on the initial ALD nucleation reactions; the ability to grow oxynitrides; and the use of simulations as a computational spectroscopy tool to compliment experiment in the determination of ALD intermediates and products.