The studies summarized above help us understand swings in prices such as those observed two years ago. Since then, regulators have mandated changes in market operations that should make the occurrence of crashes less likely (see, for instance, the NASDAQ OMX Group, Inc.’s list of current regulatory initiatives). Particularly important is the adoption of circuit-breakers at the individual security level, the establishment of risk controls when brokers provide customers with market access, and the prohibition of markets makers’ stub quotes. The challenge facing researchers and policy makers is to understand how such changes will affect the behavior of market participants, whether they will be enough to reduce financial markets fragility, and which additional policy measures, if any, are still needed.