Using the techniques of EPR and optical irradiation in conjunction with optical-absorption measurements, we have gained useful information about the nature of some hole centers in SrTiO3. Primarily, we have investigated, at temperatures near 77°K, single crystals doped with aluminum. Two principal centers have been explored: (1) the Al-O- center-a hole shared among the oxygens which surround Al3+, and (2) the X-O- center-a hole shared in a similar fashion, but more deeply trapped by a charge defect of unknown origin. The Al-O- center arises after band-gap irradiation and is characterized at 77°K by the following g values and hyperfine constants: gII=2.0137, g⊥=2.0124, AII=8.3×10-4 cm-1, and A⊥=7.6×10-4 cm-1. The X-O- center is present before optical irradiation. No hyperfine structure is observed, but the isotropic g value is 2.0130 at all temperatures from 4.2 to 300°K. An 800-nm absorption band arising after band-gap excitation has been correlated with the Al-O- center. 430- and 600-nm absorption bands have been correlated with the absence of Fe3+ in the EPR spectrum, and a 500-nm band has also been observed. In addition, the role of iron in the photochromic processes of SrTiO3 is presented. Finally, theoretical work utilizing the molecular orbital σ and π states in Oh symmetry has provided a firm basis for the sharing model. The experimental g values and hyperfine constants are discussed in light of this model and are found to be in good agreement.