We examine the importance of indexing, industry, and broad market forces in driving common effects in order flow, returns, and trading costs. Common effects are strong for order flow and returns in a sample of S&P 500 stocks, but are weak in a sample of non-index stocks and for trading costs in both samples. Industry and broad market effects exist in order flow for both samples, but indexing effects are dominant. Correlated order flow drives common effects in returns and, to a lesser extent, those in trading costs. An event study of the effect of index addition on order flow and return comovement reinforces these conclusions. Our results show that common effects are not pervasive and have implications for diversification strategies and price formation models.