Ground-state catalysis with palladium salts and complexes is a ubiquitous tool for the organic chemist. While the majority of these transformations are proposed to proceed through traditional Pd 0 /Pd II catalytic cycles, there are notable exceptions in which palladium promotes the formation of organic radicals in the ground state. As early as 1985 it was known that common Pd 0 -phosphine complexes absorb light in the visible region and possess long-lived excited triplet states, but in the last decade visible light-driven palladium catalysis has drawn significant attention. With palladium complexes acting as photocatalysts that harvest photons, and subsequently reacting through their excited triplet states for bond-breaking or bond-forming steps, these transformations are quite distinct from those employing non-Pd-based photocatalysts. Reports in this area have primarily focused on the activation of organic halides, principally alkyl halides, fuelled by the inherent problems of employing these substrates using ground-state palladium catalysis, namely slow oxidative addition rates and facile -hydride elimination from Pd II -alkyl intermediates. Since Gevorgyan's seminal 2016 report on the remote desaturation of silyl ethers, many groups have employed aryl halides 1 as precursors to aryl radicals using photoexcited palladium catalysis for For alkyl halides, a barrierless single electron transfer (SET) to form the hybrid species has been elucidated by computation, but such calculations are absent for aryl halides. Once formed, the radical can react through a variety of pathways such as hydrogen atom transfer (HAT) or addition to unsaturated species, resulting in migration of the Pd-radical hybrid (Scheme 1B). Indirect evidence for the Pd-radical hybrid is provided based on traditional mechanistic tests for radical intermediates such as radical traps, radical clocks, and deuterium labelling experiments. In addition to facilitating oxidative addition, visible light has been shown to accelerate other aspects of palladium catalysis such as reductive elimination, or reduction of the Pd II pre-catalyst. The development of the recent radical chemistry contrasts with traditional non-radical based Pd coupling reactions. For example, coupling of aryl halides 1 with arenes mediated by Pd complexes without intervention of radicals leads to formation of biaryls and this can occur by a number of routes (Scheme 1C). Oxidative addition of an aryl halide 1 to form a Pd(II) complex 2 could in principle lead to addition to an arene to form 3, followed by rearomatisation to biaryl 4. Alternatively, concerted metalation-deprotonation (CMD) could generate a complex 5 that affords the biaryl product 4 through reductive elimination. Returning to radical chemistry, aryl radicals are exceptionally reactive species and thus are difficult to identify directly as reaction intermediates. We recently reported an assay reaction using a special substrate to identify aryl radicals unambiguously. When aryl radicals are produced, the assay provides very strong mechanistic evidence through two channels -(i) a diagnostic ratio of two coupled products is observed (as is outlined in the paragraph below), and (ii) the isotope effects seen in deuterated versus non-deuterated solvent strongly either support or refute radical mechanisms (this will be discussed later in this paper). In this way, this assay provides multi-point evidence that can support or refute evidence of radicals.