This dissertation examines the impact of institutional (and individual) trading on stock prices in China. Previous literature suggests three alternative hypotheses for this impact: price pressure, informed trading, and momentum trading, but has so far not been able to distinguish between them. Using a unique dataset that contains detailed daily institutional and individual ownership information for all Shanghai Stock Exchange stocks in China, I am able to examine the important relation between daily aggregate institutional (individual) trading and past, contemporaneous, and future stock returns at a daily level. I find strong evidence of price pressure, informed trading, and momentum trading of institutional investors. These findings have important implications for the efficiency of the financial market.