Biostratigraphic study of marine Cenozoic rocks of the Pacific Northwest began some 75 years ago, in part as a consequence of the discovery of petroleum in neighboring California during the late 1800s. It was preceded, however, by an almost equally long period of reconnaissance and early description that began during the middle 1800s. Paleontological study of the marine Cenozoic was initiated by T. A. Conrad’s descriptions of Oligocene and Miocene fossils from Oregon. The first biochronologies were developed by C. E. Weaver and, to a lesser extent, by Ralph Arnold and Harold Hannibal during the early 1900s. Their molluscan-based horizons and formations formed the principal means of age determination and correlation until the mid-1900s when the California-based benthic foraminiferal zonations were adapted to the Pacific Northwest Province by Weldon Rau. Recent development of worldwide planktonic microfossil zonations has created a renewed interest in biostratigraphic study of the Pacific Northwest in addition to a means of calibrating the benthic chronologies.