Coumarin derivatives form a foundational class of blue-green fluorophores (λem 400-500 nm) derived from natural and synthetic sources. Their relatively small molecular size minimizes steric hindrance in bioconjugates. Exhibiting large Stokes shifts and environment-sensitive emission, they serve as versatile scaffolds for enzyme substrates, membrane probes, and laser dyes. Functionalized coumarins demonstrate particular utility in FRET pairs, HTS assays, and polarity sensing. Their synthetic accessibility enables customized modifications for specialized photophysical and biochemical applications.