One of Julian Simon's last works-in-progress--cut short just before completion by his death in early 1998--The Great Breakthrough and Its Cause explores the question of why human progress accelerated in Western Europe starting around 1750. Why did life expectancy, the household consumption level, speeds of travel and communication, the literacy rate, and other aspects of the standard of living leap above those in the previous centuries and millennia? What forces caused this extraordinary development to occur when it did--or even to occur at all--rather than centuries or millennia earlier or later? Simon answers this question by arguing persuasively that the total quantity of humanity--and the nexus of human numbers with technology--has been the main driving force behind what he calls "Sudden Modern Progress." Further, he continues, if population numbers had risen more rapidly than they did, the "Great Breakthrough" would have occurred earlier. He also asserts that institutional changes, phenomena often credited for human progress, are from a very long-run perspective a result of population growth. And finally, he seeks to refute two seeming counterexamples, China and India, that reached high population densities prior to the modern period without accelerated growth in consumer welfare. In his inimitable style, Simon meticulously backs up his arguments with extensive use of a wide variety of data. Along the way, he also takes on the arguments of other writers on the subject of population growth and progress, such as Joel Mokyr and Eric Jones. Completed and polished by Timur Kuran, this exploration into the great explosion of consumer welfare will stimulate, challenge, and foster high-level intellectual debate on the question of human progress. It will be of particular interest to demographers, economic historians, and a broad array of social scientists. The late Julian Simon was most recently Professor of Business Administration, University of Maryland, College Park.