In natural exposures and drill cores from rocks and oceanic sediments at several localities throughout the world, the Pliocene-Pleistocene boundary is based on evidence of climatic cooling followed by conspicuous fluctuation. The data consist mainly of marine-invertebrate fossil faunas, oxygen-isotope thermometry ratios, and terrestrial plant remains. At most localities there is evidence for transition from warmer to cooler climate rather than an abrupt break. In Italy the marine strata at the boundary grade by facies change into continental strata with a distinctive Villafranchian mammal fauna. Although not exposed, similar gradation is inferred in North Sea coastal areas. In the southern Alps the mammal fauna is closely related to sediments with pollen reflecting climatic cooling and fluctuation. Although its own climatic implications are still obscure, the fauna itself is traceable across Eurasia to the Pacific Ocean. Curves of temperature fluctuation near the Pliocene-Pleistocene boundary, for cores from the Pacific are similar to those from the Alps. Thus a framework for correlation of a boundary based on climate is beginning to emerge. Strata in North America are not yet correlated with those elsewhere; their future correlation may be dependent on radiometry. The steepness of time transgression of the climatic events may have to be determined by radiometry rather than by organic evolution, because climatic change seems to have been rapid relative to the rate of organic evolution. Radiometry does not yet afford a good check on the agreement in time between occurrences of the boundary at various localities.