Robert Almgren Quantitative Brokers and NYU We will discuss use of a market simulator to develop execution algorithms in futures and cash bond markets. The simulator incorporates particular features of these markets such as pro rata matching, cointegration, workups, implied quoting, and event dynamics. We compare executions obtained in the simulator with real executions across more than a year of real trading, to identify the main sources of discrepancy. We discuss how short term trading signals can be evaluated in the simulator.