Increasing institutional equity ownership has been one of the major trends in the U.S. stock market over the last three decades. Such a process can have important implications for both investors' welfare and properties of security prices. This dissertation investigates the investment performance and trading behavior of a comprehensive group of financial institutions including banks, insurance companies, mutual funds, independent investment advisors, and corporate and state pension funds based on quarterly disclosures of their equity-portfolio holdings.The first dissertation essay analyzes the investment performance of institutional investors. Existing research has predominantly focused on mutual funds although they still represent a relatively small proportion of the pool of institutional investors. This part of the dissertation furthers the existing research on mutual funds of Jensen (1968), Ferson and Schadt (1996), Gruber (1996), Carhart (1997), Daniel, Grinblatt, Titman, and Wermers (1997) among many others, and presents a comprehensive comparative performance analysis of institutional investors. The major finding is that on aggregate, financial institutions exhibit significant abnormal performance before expenses are subtracted. This result is robust with respect to several conditional and unconditional performance measures based on factor- and characteristic-benchmarks. Institutional performance is further related to the size of the managed portfolio, its stock characteristics, and its flows.Next, I find that performance varies across types of institutions - banks, independent investment advisors, and mutual funds significantly outperform insurance companies and state pension funds. This pattern is not caused by systematic mispricings by the applied asset pricing models. The variation of performance across the major institutional types is consistent with agency theory (see Starks (1987), Admati and Pfleiderer (1997), Stoughton (1993), Lakonshok, Shleifer, and Vishny (1992a) and the "transparency" hypothesis of Ross (1989).The second dissertation essay is presented in Chapter 3 and investigates the intertemporal trading patterns of US institutional investors. I demonstrate that institutions tend to increase (decrease) their holdings in a stock after periods of significant aggregate institutional net buys (sells). Portfolio managers are also more likely to open a position in a stock after periods of increased aggregate institutional buys. There is a substantial variation in the tendency of institutional investors to follow aggregate trades across stocks. Institutions pay more attention to their peers' trades in small volatile stocks and in stocks experiencing significant price decreases. Institutions with the highest tendency to follow aggregate institutional trades do not realize significant abnormal performance and they significantly underperform the group of contrarian institutions, betting against the trend in aggregate institutional trades.Institutional types differ in their trading decisions - banks, insurance companies, and investment advisors are more likely to act in conformity with their peers, corporate and state pension funds are not significantly influenced by previous institutional trades, while mutual funds exhibit contrarian behavior. Institutional investors further exhibit the tendency to target market weights. The results relate to a series of theoretical findings about herding and strategic trading in capital markets.