As the fastest bioorthogonal reaction, tetrazine ligation is widely utilized across chemical biology, medicine, nuclear medicine, and materials science. This talk will present the development of thioalkyltetrazines as multipurpose functional groups that serve both as practical synthetic building blocks for traditional tetrazine reagents and as chemo- and site-selective probes for protein modification, protein–protein conjugation, and cellular profiling. The talk will also discuss Catalytic Activation of Bioorthogonal Chemistry (CABL)—a strategy for "turning on" rapid bioorthogonal chemistry in cells and in vivo via the oxidation of dihydrotetrazines to tetrazines. Enzymatic catalysts and photocatalysts featuring dye-selective activation across blue, green, red, and near-IR wavelengths will be described. Kinetic studies elucidate the mechanism of CABL’s rapid kinetics and biocompatibility, which stems from chemoselective dihydrotetrazine oxidation rather than singlet oxygen generation. Kinetic studies have been used to develop a proposal explaining CABL's rapid kinetics and biocompatibility, attributed to chemoselective dihydrotetrazine oxidation rather than the production of reactive oxygen species.