The purpose of this thesis is to undertake an analysis of the determinants of aggregate flow in the US mutual funds market. The thesis comprises a series of empirical studies that focus on the relationship between aggregate flow and both market returns and mutual fund returns. In the existing literature, aggregate (market-level) studies have found a relation between flow and market returns (Warther, 1995; Potter, 2000; Luo, 2003). At the individual fund level, the literature has found that, at least for retail funds, fund return is the most important factor that investors consider when choosing between funds (Capon, Fitzsimons and Prince, 1996; Wilcox, 2003). However, to date there has been no research into the joint impact of these two sources of return on aggregate flow. This thesis addresses this gap in the literature by examining if, at the aggregate level, investors consider both market returns and also excess fund returns when deciding whether to invest in mutual funds. The analysis also tests whether the determinants of aggregate flow are homogeneous across all types of mutual funds. Hence, funds are classified as retail or institutional, depending on which investors they are open to. To date, studies of aggregate flow have not analysed retail and institutional funds separately, whereas results from studies of individual flow indicate that there are differences in the relation between flow and fund performance for the two groups of funds (James and Karceski, 2006). The latter literature suggests that these differences are attributable to institutional investors being more sophisticated and having better access to information than retail investors. As a further test, funds are also categorised into their respective investment styles. Although there has not been much research into the issue, there is some evidence that style affects flow (Potter, 2000; Karceski, 2002; Luo, 2003). The sample of funds is categorised into one of three styles, namely aggressive growth, growth, and growth & income, to investigate whether there are differences in the relation between aggregate flow and returns for different fund styles. The results show that for retail funds, both excess fund returns and market returns are determinants of aggregate flow. When the retail sample is disaggregated by style, for each style, aggregate flow responds not only to market returns, but also to the excess returns of funds of the same style. Further, there are differences in the way investors within each style respond to excess fund returns. Aggregate flow to aggressive growth and growth funds has a significant positive relation with the excess returns of the best performing funds but not the worst performing funds. In contrast, aggregate flow to growth & income funds has a significant relation with the worst performing excess fund returns but not the best. This is consistent with growth & income fund investors being the most risk averse of the three groups and consequently more sensitive to poor fund performance. Results from the analysis of institutional funds are different from the retail sample results. Neither excess fund returns nor market returns are significant determinants of aggregate flow to institutional funds. Further, no relation is found between flow to each style of fund and market or fund returns. These results suggest that whereas retail investors appear to react to relatively simple price-based signals, institutional investors use more sophisticated measures to determine whether to invest in mutual funds. The findings indicate that the extant literature has omitted a number of important pieces of information when analysing aggregate flow. First, the determinants of aggregate flow to retail and institutional funds are different, therefore the two groups of funds should not be analysed together. Second, both market returns and excess fund returns appear joint explanators of aggregate flow to retail funds and hence both variables should be included in future analysis. Third, the relation between aggregate flow and excess fund returns is not homogeneous across retail funds, so funds of different styles should be analysed separately.