To close, we have presented a transfer hydrogenation (TH) approach to activity-based sensing (ABS) of formate in living cells. We have developed a two-component ABS system consisting of a metal catalyst and fluorescent dye, in which the metal complex utilizes formate as a reductant to catalyze the conversion of an aldehyde-functionalized fluorescent dye to its alcohol counterpart with a concomitant fluorescence response. Owing to the generality of this ABS approach, and in particular the flexibility of mixing and matching catalyst and dye components to tune fluorescence outputs, we have developed formate reporter systems that operate using either an intensity-based turn-on response with a single excitation/emission signature or a ratiometric response with a dual excitation/emission shift for self-calibration, respectively. In-depth evaluation of electron transfer properties in fluorescein-like fluorophore scaffolds afforded a pair of dyes where formate-mediated aldehyde-to-alcohol conversion gave a significant 11-fold fluorescence turnon response, which was then used to systematically screen several classes of iridium complexes for optimizing formate-mediated transfer hydrogenation catalysis in aqueous solution.