Carbon-supported metal catalysts, ranging from isolated single atoms and subnanometer clusters to nanoparticles, are widely used in heterogeneous catalysis because carbon supports offer large surface area, conductivity, tunability, and abundant defects. However, their catalytically relevant structures are often local, heterogeneous, and dynamically evolving, making ensemble-averaged characterization insufficient for establishing reliable structure–activity relationships. In this context, electron microscopy has emerged as an essential tool for directly visualizing metal dispersion, coordination environments, and metal–carbon interfaces across multiple length scales. The combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, energy-dispersive X-ray spectroscopy, and complementary spectroscopies enables the identification of atomically dispersed sites, clusters, and confined nanoparticles. In addition, in situ and operando microscopy reveals catalyst formation, restructuring, and deactivation under working conditions. Future advances will rely on realistic operando cells, statistically robust analysis, and artificial intelligence–assisted workflows, transforming electron microscopy from a descriptive imaging tool into a quantitative platform for understanding and designing carbon-supported metal catalysts.