Imaging chemical reactions under synthetically relevant conditions can reveal mechanistic information that is inaccessible with traditional analytical techniques. Our laboratory develops fluorescence microscopy methods—including fluorescence intensity and fluorescence lifetime imaging microscopy (FLIM)—to understand chemical reactivity and physical processes in complex reaction media with spatial and temporal resolution. We examine aqueous–organic reactions, oxidative addition to metal powders, and catalytic polymerization. These systems present particular challenges to traditional characterization methods: aqueous–organic emulsions are heterogeneous and optically opaque, organometallic surface intermediates in oxidative–addition reactions do not substantially build up, and many growing polymers are insoluble or spatially heterogenous. In these systems, FLIM reveals object sizes, catalyst localization and environments, the role of reagents, and the physiochemical reasons underpinning catalytic turnover rates. For example, droplet-to-droplet differences in emulsions under cross-coupling conditions suggest that individual droplets function as distinct reaction vessels.