**Abstract**

Important areas such as environmental sciences, medicine, pharmacy, and cellular biology are dependent on very sensitive analytical techniques. One of the most common methodologies used for their bioanalytical purposes is the fluorescent labelling. The synthesis of new fluorophores and the great development of fluorescent-labelling techniques combined with the enormous technological advances in the field of fluorescence microscopy allowed to deepen the structural knowledge of biomolecules. This new organic fluorophores form covalent bonds with the sample to be analyzed, producing stable bioconjugates that show fluorescence in a wide range of wavelengths, depending on the label used. Coumarin derivatives represent one of the most important chemical classes of organic fluorescent materials being one of the most extensively investigated and commercially significant groups of organic fluorescent materials. In this chapter, it is reviewed the use of fluorescent coumarin derivatives and their application to labelling biomolecules. These fluorescent labels allow researchers to study, and understand, biomolecular assemblies that exhibit complex sensitivity and selectivity. Reactive fluorescent coumarin derivatives are actually widely used in labelling biomolecules as peptides, proteins, oligonucleotides, nucleic acids, and carbohydrates, among other biological molecules.

**Keywords:** coumarins, fluorophores, labelling, biomolecules, bioconjugation
