We provide new evidence on the relation between order flow and prices, an issue that is central to asset pricing and market microstructure. We examine proprietary data on a broad panel of NYSE-listed stocks that reveal daily order imbalances by institutions, individuals, and market makers. We can further differentiate regular institutional trades from institutional program trades. Our results indicate that order imbalances from different trader types play distinctly different roles in price formation. Institutions and individuals are contrarians with respect to previous-day returns, but differ in the effect their order imbalances have on contemporaneous returns. Institutional imbalances are positively related to contemporaneous returns, and we provide cross-sectional evidence that this relation is likely to be the result of firm-specific information institutions have. Individuals, specialists, and other market makers appear to provide liquidity to these actively trading institutions. Our results also suggest a special role for institutional program trades, which tend to be uninformed and provide liquidity to more aggressively trading institutions. Finally, institutional non-program imbalances (information which is not available to market participants) have predictive power for next-day returns.