Abstract This chapter presents an overview of the distributional patterns of the major calcareous and siliceous microfossil groups of the Paleogene in the western North Atlantic Ocean. From these patterns the evolution of open marine biofacies through space and time can be reconstructed. At first sight, this would seem to be an easy task, considering the large number of Deep Sea Drilling Project (DSDP) and commercial drill holes available, as well as the numerous piston cores which have retrieved pre-Quatemary sediments from the North Atlantic Basin in the last 25 years. However, two principal problems complicate detailed biofacies reconstructions. The first, and most frustrating, is the incompleteness of the Paleogene Stratigraphie record from the North Atlantic. Existing deep-sea cores are complicated by extensive drilling gaps, large sedimentary hiatuses, and/or the absence of age- diagnostic fossils. The second problem is the difficulty of intercorrelation of the many different Paleogene biostratigraphic zonations and time-scales. Since about 1969, biostratigraphic schemes based on the major calcareous and siliceous fossil groups have been extensively refined or newly constructed. Because different biostratigraphic schemes and taxonomies have been used by different researchers in earlier studies of North Atlantic cores, the majority of Paleogene calcareous DSDP sections and many of the other boreholes from the western North Atlantic have been restudied and rezoned during preparation of this chapter. The sections have all been recalibrated according to the time scale and zonation shown on Plate 1. As a result, much of the early Paleogene can now be accurately correlated and