Based on CASPT2 results, the present contribution establishes for the first time that cytosine photodimer formation (C< >C) is mediated along the triplet and singlet manifold by a singlet-triplet crossing,(T-1/S-0)(X), and by a conical intersection, (S-1/S-0)(Cl), respectively. The former can be accessed in a barrierless way from a great variety of photochemical avenues and exhibits a covalent single bond between the ethene C-6-C-6' carbon atoms of each monomer. The efficiency of the stepwise triplet mechanism, however, would be modulated by the effectiveness of the intersystem crossing mechanism. The results provide the grounds for the understanding of the potential photogenotoxicity of endogenous and exogenous compounds via triplet-triplet sensitization, with a lower bound for cytosine oligonucleotides predicted to be 2.70 eV, and give support to the traditional view of the primary role of triplet excited states in the photochemistry of DNA, a well-known source of photoproducts in solution under triplet photosensitization conditions. The function played by singlet excimers (excited dimers) to explain both the red-shifted fluorescence and photoreaction is highlighted. A rationale on the pronounced wavelength dependence of the observed fluorescence is offered. Geometrical arrangements at the time of light irradiation close to, but energetically above, (S-1/S-0))(Cl) are suggested as reactive orientations that become prone to produce C< >C directly, with no energy barrier. Because of the outstanding intrinsic ability of cytosine to form stable relaxed excimers, the system located near the bound relaxed excimer has to accumulate enough vibrational energy to surmount a small barrier of 0.2 eV to reach (S-1/S-0)(Cl), making the overall process to proceed at a slower relative rate as compared to other compounds such as thymine, which is not susceptible of forming so stable excimers.