NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Base pressure measurements of circular cylinders at Reynolds numbers from 100 to 3 x [...] were obtained in a low speed wind tunnel. Pressure distributions from [...] to [...] were also obtained. In the Reynolds number region from the first appearance of an unsteady wake to the critical Reynolds number the base pressure coefficient shows two maximums and two minimums. The first maximum (Rd [...] 50) coincides with the first appearance, of the Karman vortex street. The succeeding minimum (Rd [...] 300) probably corresponds to the movement of the onset of transition from the end of the vortex-formation region into the free shear layers. The following maximum (Rd [...] 2200) is accompanied by a small discontinuity in the base pressure curve and a decrease in the Strouhal number. The final minimum corresponds to the end of the Schiller-Linke region, at which point transition to turbulence occurs in the free shear layers very near to the shoulder of the cylinder. A comparison of the blockage correction theories of Maskell and Allen and Vincenti was made at [...] = 6 x [...]. The value of the base pressure coefficient in an unlimited stream at this Reynolds number was found to be [...] = -1.215 for cylinders of one particular roughness, though this value depends on other unit Reynolds number effects. These effects can also result in a significant Reynolds number shift of the base pressure coefficients.