The present invention relates to debugging of computer programs, and in particular to bi-directional debugging.A method of returning to a state in the history of execution of a computer program, said state comprising a set of values of one or more of registers of a processor on which the program is running, working memory space to which the program has access and operating system resources allocated to the program, the method comprising: identifying in machine code representing said program, instances of machine code instructions associated with substantially non-deterministic events; modifying said program machine code to execute a program instrumentation set of machine code instructions to handle said substantially non-deterministic events; executing said modified program machine code, storing a time series of said states during said executing; restoring a said stored state; and executing said modified program machine code forward in time starting at said restored state to return to said state in said program history of execution.