Metathesis reactions represent a mild, and complementary synthetic approach towards constructing complex molecules and polymers. The ability to run the reaction in the forward or reverse direction to construct or deconstruct the bonds using the same catalytic system are attractive features which allows for reversible reorganization of molecular fragments that are not possible using conventional reactions. Olefin metathesis (Figure ) is one of the most prominent examples of reversible metathesis reactions, and has led to exciting applications such diverse fields as biomass valorization, commodity and fine chemical synthesis, waste upcycling, supramolecular assembly, and recyclable polymer synthesis. Using the metathesis strategy, our group and others have developed catalytic functional group metathesis reactions between two different reactive functional groups. However, the broader applicability of these new methods, as well as the established applications above, is clearly limited as two preinstalled reactive functional groups are required on both reaction partners. On the other hand, C-H functionalization makes use of ubiquitous C-H bonds to directly form elaborated products without the need for prefunctionalization. A reversible metathesis approach to C-H bond functionalization would thus merge two attractive concepts and provide exciting prospects for synthetic chemistry, however, this approach has remained considerably underexplored. Among the rare examples of C-H bonds being used in reversible metathesis reactions, the most notable are C(sp 2 )-H to C(sp 2 )borylation/iodination where self-correcting abilities have led to interesting selectivities. In addition to these methods, an important industrial application of C-H metathesis is the C-H/C-Me metathesis of toluene to form xylenes and benzene. Given these precedents, it would be particularly attractive to incorporate native C(sp 3 )-H bonds into metathesis strategies to expand the capabilities of C-H functionalization, and enable a new class of metathesis reactions (Figure ). Using the conceptual appeal of a C-H bond metathesis (Figure ,D), we believed a C(sp 3 )-H benzylic halogenation would be attractive and complementary to traditional methods. Modern reagents have been developed for unactivated C(sp 3 )-H halogenation, but benzylic C(sp 3 )-H positions still largely rely on classical reagents. These reagents (X2: X = Br or Cl, or AIBN and NBS/NCS) often lead to overhalogenation, and require tailored optimization for individual substrates. The reverse reaction, i.e. dehalogenation, cannot be achieved using these methods and requires different conditions which commonly employ toxic tin reagents, although modern methods offer some alternatives. "Overhalogenation" is another common problem associated with classical methods where a metathesis approach could offer the prospect to regenerate a desired monohalogenated product by reacting the overhalogenated material in reverse. First, we sought to identify an HAT catalyst and XAT catalyst which could reversibly cleave and form a C-H and C-X bond and proceed through a common intermediate, in this case a free radical. Here a diverse range of catalysts that have been utilized in deracemization, epimerization, functional group migration, and "radical sampling" 53 could be surveyed. Reversible halogen atom transfer (XAT), as employed in controllable radical polymerization like atom transfer radical polymerization (ATRP) serves as a platform for evaluating different XAT catalysts. Despite separate applications of reversible HAT and XAT, identifying two catalysts that can simultaneously, yet independently, activate a C-H bond (through HAT) and a C-X bond (through XAT) has remained challenging. This is likely due to the challenges of matching the kinetics of both cycles, and preventing the catalysts from interfering with each other. We began our investigation by surveying a panel of Culigand combinations commonly employed in ATRP in combination with HAT catalysts (Figure ). Despite reports of dual catalytic cycles with decatungstate (DT) and copper, only dehalogenation and radical dimerization were observed clearly demonstrating the challenges of matching activation kinetics and catalyst compatibility. We then shifted our focus to aryl ketone HAT catalysts. After evaluating several HAT catalysts (entries 2-3), 5,7,12,14pentacenetetrone (PT) proved uniquely effective for this transformation (entries 4-8). The preference of excited state PT to undergo HAT instead of electron transfer, or energy transfer pathways led the success of this catalyst and allowed us to overcome the above-mentioned challenges (vide supra). To test the efficacy of the system for selective halogen sorting, a mixture (~7:1) of monohalogenated 3a (0.50 mmol), dihalogenated 3a' (0.075 mmol), and non-halogenated 1a (0.15 mmol) were subjected to the optimized reaction conditions. After full conversion of 3a', 3a (0.57 mmol) was obtained in 86% yield (yield based on mols of Cl). Besides its mechanistic value, this experiment also indicates the possibility to use halogen sorting to convert a mixture of polyhalogenated isomers to a single product. We propose that the HAT and XAT catalysts are working in concert to predictably and selectively control the metathesis products. The HAT catalyst operates under kinetic control (consistent with the observed primary KIE of 1.9, Fig. ) and cleaves the most electron-rich sterically accessible C-H bonds in the forward direction. In the reverse direction, it prefers to form electron-poor C-H bonds through proton and electron transfer to electronpoor radicals. The XAT catalyst can activate both electronpoor and electron-rich C-X bonds, although activation of electron-poor C-X bonds is faster. The electron-poor Cu-II intermediate then combines faster with the more electronrich radical over the electron-poor cyanomethyl radical. The activation of all C-X bonds is fully reversible, however due to other irreversible or slow steps in the HAT cycle, benzylic C-X/C-H bonds can only exchange very slowly (See supplementary information for benzyl C-X/C-H exchange control experiments). Running the reaction in the forward direction, simple benzyl and napthyl deratives (2ab; 3a-b)and 2c-e and 3c-e which contain typical crosscoupling handles could be halogenated in good yield. Secondary C-H bonds (1f-i) also worked in good yield for bromination and chlorination, although attempted bromination of cyclic systems led to desaturation forming styrenes (2f). The unprotected drug molecule Ibuprofen provided the desired product 2h and 3h with complete selectivity for the more electron rich benzylic C-H bond, and no competing decarboxylation. Protection of Ibuprofen as the methyl ester increased the yield significantly for bromination product 2i. An electron-donating group in the para position worked well for chlorination (3j), although the brominated product 2j was lower yielding and unstable towards isolation. Ortho substituents on 1k-l resulted in lower yields for both bromination and chlorination with significant amounts of unreacted started material. Strong electron withdrawing groups and electron-poor heterocycles (pyridines, or Ar-NO2/-CF3) in any position of the ring completely shut down the reaction suggesting the C-H bonds are too electron-poor to be activated. The use of the bulky PT catalyst allowed us to overcome classical selectivity of C-H halogenation and preferentially functionalize the sterically most accessible C-H bond over the weaker C-H bond. P-cymene 1m, gave complete selectivity for the methyl group, which contrasts with the selectivity of NBS or Cl2 . The methyl group could even be discriminated over a secondary benzylic position, giving a modest selectivity of 1.7:1 for 1n. This inverts selectivity trends observed with modern methods which give >20:1 selectivity for secondary bromination or 2.4:1 for secondary chlorination. When applying the conditions to unactivated aliphatic C-H bonds, lower yields were obtained when using one equivalent of C-H starting material. This is consistent with previous reports where PT is employed, and is consistent with Stern-Volmer quenching experiments showing less-efficient quenching of the excited photocatalyst by aliphatic substrates compared to benzylic substrates (See supplementary information). Using an excess of C-H partner, cyclooctane gave good yields to form 2r and 3r, and adamantane gave good yields with complete selectivity for 1-halo adamantanes 2s and 3s. Using the natural products (+)-sclareolide, and isosteviol methyl ester which are benchmarked substrates for selectivity in C-H HAT processes, we obtained 2t/3t and 2u/3u. Although 2t/3t are known in the literature, 2u/3u have not been reported, and the regioselectivity of the product 2u was unambiguously assigned by X-ray diffraction analysis. The use of the C-H metathesis reaction was then evaluated as a tool for transferring halogens from halogenated persistent organic pollutants in an upcycling reaction. With millions of tons of chlorinated POPs remaining stockpiled, current protocols for their disposal involve expensive transport to specialized incinerators, thereby making halogen upcycling an increasingly attractive approach. Using DDT or lindane as the limiting reagent, toluene (1a) could be chlorinated in high yield giving 3a, while aliphatic substrates such as adamantane (1s) provided moderate yields of 3s. Utilizing brominated flame retardant HBCD as a brominating source, cyclooctane provided moderate yields, however, the benzylic bromination performed significantly worse than the chlorinated POPs. In this case, we attribute these low yields to strong product inhibition of 2a which may react with the Cu-I orders of magnitude faster than HBCD thereby shutting down the reaction. Similar to other reports which report incomplete conversion of halogenated pollutants, this strategy may be use in synthetic chemistry, but challenging to employ as a large-scale remediation strategy.