Introduction Complex reaction networks consisting of hundreds of different species are common in many disparate types of reaction, including oxidation, pyrolysis and polymerization chemistries, with the complexity arising through the presence of many reactants or highly reactive species. In contrast, the complex reaction networks of transition metal catalyzed processes originate from the nature of catalysis itself, wherein there exists an interplay of distinct elementary steps that must all occur simultaneously within a single environment. Often multiple productive catalytic cycles compete with side reactions and catalyst decomposition pathways. Furthermore, variation in (pre)catalyst, ligand, additive, solvent, and reaction stoichiometries, order of mixing and temperature (amongst other variables) can result in dramatic changes in catalytic speciation and hence reaction outcome(s). Catalyst speciation adds considerable complexity, an aspect that is largely ignored in chemical synthesis, with the focus placed on the product(s) yield. Such complex reactions are often met with trepidation by synthetic chemists, with too many variables seemingly outside of their control. Indeed, most reactions under investigation in synthetic chemistry laboratories target selective formation of a single dominant product in high yield (i.e. an idealized outcome). However, there are opportunities to be had through embracing complexity. Firstly, complex reaction manifolds provide an opportunity for serendipitous reaction discovery, and complex reactions can often lead to the formation of unexpected high value products and new reaction chemistries. Hartwig and co-workers elegantly demonstrated how complexity could be deliberately employed for discovery. Subjecting complex mixtures of reagents to potential catalysts resulted in the discovery of new reactions. Secondly, developing an understanding of a complex reaction and the sensitivities to reaction conditions allows management of side-product profiles, particularly with final product purity and potential toxicity in mind. This is of vital importance in industrial process development where reliability is critical. Thirdly, studying complex reactions can lead to new mechanistic understanding through the identification of competing reaction pathways and interconnected catalytic cycles, that can be used to direct and support traditional mechanistic studies, i.e. hypotheses built on firm foundations involving real-world substrates and side-product profiles.