The emitting light in fireflies arises from the electronic relaxation of oxyluciferin, an organic compound resulting from the oxidation of the D-luciferin substrate inside an enzyme called luciferase. In order to theoretically study such systems, the use of quantum mechanical/molecular mechanical (QM/MM) methods is required. Accurate QM level is needed for dealing with electronic transition and charge transfer phenomena. Taking into account the surrounding protein at the MM level is essential in order to understand the colour modulation and influence of the enzyme. The present lecture will discuss some of the results obtained on this system with calculations done with the coupling of the programs MOLCAS (CASPT2/CASSCF) or G09 (DFT) and TINKER (AMBER force field) and how the complementary of experimental data and theoretical results can give understanding of such phenomenon. The transition energy corresponding to a fluorescence experiment for the wild type emitter and a modified emitter will be presented. Calculations give insights to decipher the emitter conformation even with a lack of experimental crystallographic structure. And insights to explain the color modulation due to the surrounding environment.