A simple framework decomposes changes in a fund family’s market share into four components. The components are highly relevant for understanding mutual fund market dynamics and evaluating the business performance of fund families. Two components are performance driven, and two are flow driven. Analysis of US market data shows that the “Excess Flows” component, which measures whether fund family flows exceed or lag those of competitors that operate in the same fund categories, has the biggest impact on fund-family market share changes. Major cross-sectional differences characterize how individual families score on each of the components. Fund families can use this framework to provide input for strategic decision making.