The activation of inert C-H bonds lies at the heart of organic chemistry. In particular C-H activation using transition metal catalysis has made a profound impact on complex molecule synthesis, but the area remains important for future discovery. At present the utility of synthetic methods based on C-H activation is hampered by the inherent difficulty of being able to selectively functionalize a single C-H bond in the presence of many others. Thus the ability to perform predictably site-selective C-H functionalizations on a given C-H bond in a complex substrate would be transformative for chemical synthesis.In this proposal we propose to perform computational studies on Pd-catalyzed C-H functionalization reactions, to uncover the inherent electronic bias of substrate structures on the site-selectivity. Calculations will be performed using density functional theory to characterize the mechanisms and catalytic cycle for Pd-catalyzed arylation of aromatic and heteroaromatic substrates. We will also develop quantitative models of reactivity and selectivity to deliver a greater understanding of the process, which will be used to generate predictions. The result will be a reliable predictive method with which to rationally design substrates and catalysts to deliver improved selectivities in C-H functionalizations.