The media are increasingly recognized as key players in ﬁnancial markets. I investigate their causal impact on trading and price formation by examining national newspaper strikes in several countries. Trading volume falls 12% on strike days. The dispersion of stock returns and their intraday volatility are reduced by 7%, while aggregate returns are unaffected. Moreover, analysis of return predictability indicates that newspapers propagate news from the previous day. These ﬁndings demonstrate that the media contribute to the efﬁciency of the stock market by improving the dissemination of information among investors and its incorporation into stock prices.