Following its widespread use in biomedical research, fluorescence microscopy has recently been introduced in the catalysis field to study chemocatalytic processes with a high spatiotemporal resolution, a unique sensitivity down to the single molecule level and this under in situ conditions. This tutorial review is structured around the length scales that are currently accessible in fluorescence microscopy and discusses the different conceptual approaches that have been developed to study molecular concentration and dynamics like diffusion and catalytic conversion at these micron and sub-micron levels.