This chapter lists the CAS registry number, chemical structure, chemical abstract (CA) index name, other chemical names, chemical/dye class, synthesis procedure, molecular formula, and molecular weight of the marina blue (6,8‐difluoro‐7‐ hydroxy‐4‐methylcoumarin) (DiFMU). Other properties listed for this fluorescent dye include the physical form, solubility, melting point, boiling point, pKa, absorption maxima, emission maxima, molar extinction coefficient and quantum yield. The chapter also includes references on the use, properties of DiFMU. It presents a list of imaging/labeling applications, biological/medical applications and industrial applications of DiFMU.