We test the prevalence of herding among large speculative traders in futures markets by employing a unique dataset from the U.S. CFTC on individual positions of these traders in thirty-one futures markets covering 2002 - 2006. We compare and contrast herding among hedge funds and floor participants and test whether herding serves to stabilize or destabilize market prices. While we find some mild evidence of herding among hedge funds and floor participants, herding levels are, on average, comparable between these groups and similar to herding levels in equity market studies. We also analyze the determinants of the herding measure and find that the number of traders and trading volume are positively associated with herding. Although hedge fund positions are also significantly related to variation in herding, we find, notably, that the moderate levels of herding in futures markets serve to stabilize prices.