We analyze data from 2005 through 2009 that uniquely identify categories of traders to assess how speculators such as hedge funds and swap dealers relate to volatility and price changes. Examining various subperiods where price trends are strong, we find little evidence that speculators destabilize financial markets. To the contrary, hedge funds facilitate price discovery by trading with contemporaneous returns while serving to reduce volatility. Swap dealer activity, however, is largely unrelated to both contemporaneous...