Firefly luciferase is one of the most studied bioluminescent systems, both theoretically and experimentally. Herein we review the current understanding of the bioluminescent process from a chemical functionality perspective based on those investigations. Three key components are emphasized: the chemiluminophore, the electron-donating fragment, and how these are affected by the substrate-enzyme interaction. The understanding is based on details of how the peroxide -O-O- bond supports the production of electronically excited products and how the charge-transfer (CT) mechanism, with the aid of an electron-donating group, lowers the activation barrier to support a reaction occurs in living organisms. For the substrate-enzyme complex it is demonstrated that the enzyme can affect the hydrogen-bonding around the CT-controlling group, resulting in a mechanism for color modulation. Finally, we analyse other luciferin-luciferase systems and compare them to the key chemical functionalities of the fragments of the luciferin-luciferase complex with respect to similarities and differences.