A method and system debug a computer program by using trace data, which is a recording of the sequence of machine instructions executed by a program during a time period along with the addresses and values of memory locations accessed and modified by each machine instruction. After the time period, the method and system use the trace data to simulate the execution of the program during the time period under the control of a debugger. In addition, the method and system use the trace data to simulate the execution of the program during the time period backwards in time under the control of the debugger.