The Middle and Upper Paleolithic periods encompass a vast time span in which a remarkable amount of human biological and cultural change occurred. The period saw not only the emergence of anatomically modem humans, but also of fully modern human behavior. Archaeologically, the appearance of fully modern behavior is usually equated with Early Upper Paleolithic assemblages from Africa, Southwest Asia and Europe. Originally viewed as a replacement of one culture by another on the basis of European evidence, the emergence of the Upper Paleolithic has been explained somewhat differently on the basis of Levantine evidence. Assemblages from several Levantine sites have been shown to contain evidence of a cultural transition from the Middle to Upper Paleolithic. The recent discovery and excavation of the rockshelter of Tor Sadaf provides a new opportunity to examine the technological changes that took place across the Middle to Upper Paleolithic transition and earliest Upper Paleolithic. Located in the Wadi al-Hasa, west of the Jordan Rift Valley, the site was excavated during 1999 and 1998. Excavations recovered more than 25,000 lithic artifacts from more than one meter of cultural deposits. Technologically and typologically, the lithic materials are diagnostic of the Middle to Upper Paleolithic transition and Early Upper Paleolithic. This study focuses on a fine-grained technological analysis of the lithic artifacts from Tor Sadaf, in an effort to understand how changes in lithic reduction strategies occurred during this critical time period in human prehistory. Analyses of cores, debitage and tools from the site are integrated to model how core reduction strategies evolved over time in response to changes in tool selectivity. Evidence from Tor Sadaf is also compared with that from other contemporaneous sites, in order to place the site in the larger context of the Late Pleistocene Levant. It is argued that the emergence of the Levantine Upper Paleolithic involved an autochthonous evolution from the Levantine Mousterian. This evolutionary change is probably related to changing mobility and subsistence strategies that occurred in the vicinity of ca. 40 thousand years ago. The analysis is also used to suggest new criteria for distinguishing between transitional and Early Upper Paleolithic assemblages.