This document summarizes the history and development of the concept of market efficiency. It discusses early works in the 1900s that anticipated the idea, and key studies in the 1950s-60s that developed the random walk model and market efficiency theory. Major topics covered include event studies in the late 1960s that provided empirical evidence; analysis in the 1960s-70s of mutual funds and managers that supported efficient markets; and anomalies identified starting in the 1970s that challenged aspects of efficiency. The document concludes by noting the ongoing debate between the efficient market framework and behavioral theories to explain anomalies.