One of the most striking changes in U.S. equity markets has been the proliferation of trading venues. My dissertation studies the impact of market fragmentation on liquidity and price discovery from three different perspectives. The first section, coauthored with Maureen O'Hara, examines how fragmentation of trading is affecting the quality of trading. We use newly-available trade reporting facilities volumes to measure fragmentation levels in individual stocks, and we use a matched sample to compare execution quality and efficiency of stocks with more and less fragmented trading. We find market fragmentation generally reduces transaction costs, as measured by effective spread and realized spread, and increases execution speeds. Fragmentation does increase short-term volatility, but prices are more efficient in that they are closer to being a random walk. The second section focuses on a particular type of new trading mechanism, crossing network, in which buy and sell orders are passively matched using the price set by the stock exchange. The results show that the crossing network harms price discovery and the relative lack of revealed information most strongly affects stocks with high uncertainty in their fundamental values. I find that an increase in the uncertainty of the fundamental value of the asset increases the transaction costs in both markets, but stocks with higher fundamental value uncertainty are more likely to have higher market shares in the crossing network. The impact of different allocation rules in the crossing network on market outcomes is also examined. The third section tests the theoretical prediction of the second essay. I find that crossing networks have lower effective spread and price impact of trade, but they also have lower execution probability and speed of trade. Non-execution is positive correlated with price impact, decreases in trading volume and increases in volatility. Crossing networks have higher market share for stocks with lower volatility and higher volume. We also find that the underlying assumption in previous literature, that stocks with higher effective spreads have higher reductions in effective spread by trading in crossing networks, is not supported by data.