This proposal details the mechanism-based discovery of ground-breaking new catalyst systems for a broad range of arylation processes that will be of immediate and long-lasting utility to the pharmaceutical, agrochemical, and materials chemistry industries. These industries have become highly dependent on coupling technologies employing homogeneous late transition metal catalysis and this reliance will grow further, particularly if the substrate scope can be broadened, the economics, in terms of reagents and catalyst, made more favourable, the reliability at scale-up improved, and the generation of side-products, of particular importance for optical and electronic properties of materials, minimized or eliminated.This proposal addresses these issues by conducting a detailed and comprehensive mechanistic investigation of direct arylation, so that a substantial expansion of the reaction scope can be achieved. At present, the regioselectivity can be very high, however catalyst turnover rates are moderate, and the arene is required to be in a fairly narrow window of activity. Specific aspects to be addressed in terms of mechanistic study are: catalyst speciation and pathways for deactivation; pathways for homocoupling; influence of anions and dummy ligands; protodemetalloidation pathways. Areas proposed for mechanism-informed development are: expansion of metalloid tolerance; expansion of arene scope; use of traceless activators and directors, new couplings via ligand exchange, the evolution of simpler / cheaper and more selective / active catalysts; expansion to oxidative double arylations (Ar-H + Ar’-H) with control, and without resort to super-stoichiometric bias.The long-term legacy of these studies will be detailed insight for current and emerging systems, as well as readily extrapolated information for the design of new, more efficient catalyst systems in academia, and their scaleable application in industry