The high intrinsic decontamination resistance of Firmicutes spores, is important medically (disease) and commercially (food spoilage). Effective methods of spore eradication would be of considerable interest in the health care and medical products industries; particularly if the decontamination method effectively killed spores while remaining benign to both humans and sensitive equipment. Intense blue light at ∼ 400 nm is one such treatment that has drawn significant interest. This work has determined the resistance of spores to blue light in an extensive panel of strains, including wild-type strains and mutants that: i) lack protective components such as the spore coat and its pigment (s) or the DNA protective α/β-type small acid-soluble spore proteins (SASP); ii) have an elevated spore core water content; or iii) lack enzymes involved in DNA repair, including homologous recombination and non-homologous end joining (HR and NHEJ), apurinic/apyrimidinic endonucleases, nucleotide- and base excision repair (NER& BER), translesion synthesis (TLS) by Y-family DNA polymerases and spore photoproduct (SP) removal by SP lyase (SPL). The most important factors in spore blue light resistance were determined to be: spore coats/pigmentation, α/β-type SASP, NER, BER TLS and SP repair. A major conclusion from this work is that blue light kills spores by DNA-damage, and the results in this work indicate at least some of the specific DNA-damage. It appears that high-intensity blue light could be a significant addition to the agents used to kill bacterial spores in applied settings.Effective methods of spore inactivation would be of considerable interest in the health care and medical products industries; particularly if the decontamination method effectively killed spores while remaining benign to both humans and sensitive equipment. Intense blue light radiation is one such treatment that has drawn significant interest. In this work in a systematic fashion all known spore protective features, as well as universal and spore-specific DNA repair mechanisms were tested on their contribution to the resistance of spores to blue light radiation.