Oxygen isotope stage 3 (OIS 3) was a mild interval between the two cold maxima of the last (Weichselian) glaciation marked by climate changes oscillating on a 100–1000 yr time scale between near-interglacial and peak-glacial conditions. During OIS 3, modern humans entered Europe, and somewhat later their Neanderthal predecessors became extinct. Our understanding of this momentous event depends on an answer to the question, Did the unstable environmental conditions of the time play a significant role in early human history? The Stage 3 Project is an interdisciplinary study with two main goals: (1) to describe with existing data and to simulate the climates and landscapes of typical warm and cold phases between 45,000 and 30,000 yr ago and (2) to compare the results with the spatial and temporal distribution of human beings in this context. This paper introduces the Stage 3 Project and provides background to a set of papers on the climate and landscape aspects of the Project that will appear in Quaternary Research and to studies of their relevance to the Early Upper Paleolithic of Europe to appear in journals yet to be determined.