This paper explores the impact of investor flows and financial market conditions on returns in crude oil futures markets. I argue that informational frictions and the associated speculative activity may induce prices to drift away from "fundamental" values, and may result in price booms and busts. Particular attention is given to the interplay between imperfect information about real economic activity, including supply, demand, and inventory accumulation, and speculative activity in oil markets. Furthermore, I present new evidence that there were economically and statistically significant effects of investor flows on futures prices, after controlling for returns in the United States and emerging-economy stock markets, a measure of the balance sheet flexibility of large financial institutions, open interest, the futures/spot basis, and lagged returns on oil futures. The largest impacts on futures prices were from intermediate-term growth rates of index positions and managed-money spread positions. Moreover, my findings suggest that these effects were through risk or informational channels distinct from changes in convenience yield. Finally, the evidence suggests that hedge fund trading in spread positions in futures impacted the shape of term structure of oil futures prices. This paper was accepted by Wei Xiong, finance.