The first essay examines cross-sectional differences between money market mutual funds (MMMFs), in the context of the sponsoring fund family. While extant studies have shown that fund family characteristics impact the management of open-end equity mutual funds, results of this study?s analysis find that fund family characteristics also affect the management of MMMF assets, contributing to differences in the maturity of the fund?s holdings, expenses, and realized returns. I find that an MMMF is not simply a transitional account with a short-term low-risk investment objective, but rather, a critical role player within the fund family. Differences in maturity, yield, and expenses in MMMFs can be explained by family-specific characteristics, including diversification and cash management strategies at the family level. The second essay examines implications of substitutability of two similar financial assets: conventional index mutual funds and exchange traded funds (ETFs). I seek to explain the coexistence of these fund types, since both offer a claim on the same underlying index return process, but have different organizational structures. This study compares conventional open-end index funds with matched ETFs on various underlying indexes. Aggregate flows are used to detect substitution and clientele effects. I show that conventional funds and ETFs are substitutes, while ETFs have smaller tracking errors and lower fund expenses. However, I find that these fund types are not perfect substitutes, and their coexistence can be explained by a clientele effect that segregates them into different market niches.