In this presentation, I will show how the theoretical studies can be used to give information on the bioluminescent system luciferase/oxyluciferin in fireflies. In simulation of such system, very accurate quantum chemistry has to be used in order to understand the electronic transition that lead to the photons emission. The modulation of the light emission is due to the protein environment. In order to take into account this environment, the surrounding is described with lower accuracy than for the emitter. The coupling of the two methods is called QM/MM (quantum mechanic/molecular mechanic). QM/MM studies will be presented for firefly oxyluciferin/luciferase[1,2] as long as for a new firefly luciferin analogue. This last molecule, synthetized and tested by experimental team[3], gives a red emission with better in vivo tissue penetration and signal quantification in bioluminescence imaging. Figure: oxyluciferin analogue (QM: sticks and balls) and luciferase (MM: ribbon) [1] I. Navizet, Y.-J. Liu, N. Ferré, H.-Y. Xiao, W.-H. Fang, R. Lindh, J. Am. Chem. Soc. 2010 132, 706. [2] I. Navizet, L. Yue, Y-J Liu, D. Roca-Sanjuan, N. Ferré, and R. Lindh, Photochemistry and Photobiology 2013, 89(2), 319-25. [3] A. P. Jathoul, H. Grounds, J.C Anderson and M. A. Pule, Angew.Chem.Int.Ed. 2014 53, 13059 –13063