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The basalts within Oceanus Procellarum have been named the Oceanus Procellarum Group and subdivided into four major geologic units on the basis of morphology, spectral reflectance, and other remote sensing information in order to develop a framework of regional stratigraphy for the largest lunar mare. The oldest mare basalt unit identified, the Repsold Formation (blue color, low albedo, high crater density, very weak 3.8-cm radar backscatter, and a weak 1-μm reflectance) crops out in NW Procellarum and may be correlative with Apollo 11 and 17 high-titanium basalts. The next youngest unit, the Telemann Formation (red, higher albedo, high crater density, high DL values, and an average to strong 1-μm reflectance) extends over wide areas of Procellarum and crops out as extensive surface units and as local patches surrounding earlier, topographically higher units. Numerous large sinuous rules in the Aristarchus Plateau were a major source for some of these deposits. Pyroclastic deposits forming the dark red mantle were also emplaced at the Aristarchus Plateau during these eruptions. The Hermann Formation (intermediate color, low albedo, intermediate crater density, and average to strong 1-μm reflectance) occurs as extensive plains developed from less voluminous eruptions from sources at the Marius Hills complex, the Rümker Hills, and in Nubium, Cognitum, and near Delisle. The youngest unit, the Sharp Formation (blue color, very low albedo, low crater density, and a strong 1-μm reflectance) is volumetrically small and is derived primarily from vents near the highland/mare boundary. Many Sharp Formation members have associated sinuous rules. Basalts of the Hermann Formation were sampled by Apollo 12; those of the Telemann Formation may be similar to the Luna 24 VLT basalts, while those of the Repsold Formation may be comparable with the Apollo 11, 17 and Luna 16 samples. The Sharp Formation, the youngest basalts, remains unsampled. The total volume of Procellarum basalts is estimated at 8.7×105 km3 (about 10% of the total mare volume). The relative volumes of the individual formations are not reflected by the surface area covered because the youngest flows tend to be thin and widespread. For example, the Sharp Formation has a surface area of 72,000 km2 but an estimated volume of only 1.8×104 km3, while the Repsold Formation has a present surface area of only 22,000 km2 but an estimated volume of 2.1×105 km3. The vast majority of lavas in Procellarum were extruded early in the phase of lava flooding, prior to about 3.5 b.y.