In 1970, in “Efficient Capital Markets: a Review of Theory and Empirical Work,” Gene Fama defined a market to be “informationally efficient” if prices at each moment incorporate all available information about future values. Informational efficiency is a natural consequence of competition, relatively free entry, and low costs of information. If there is a signal, not incorporated in market prices, that future values will be high, competitive traders will buy on that signal. In doing so, they bid the price up, until it fully reflects the information in the signal. Much of the confusion about “efficiency” reflects simple ignorance of this definition.