Abstract: Embodiments of a method of simulating, testing, and debugging of concurrent software applications are disclosed. Software code is executed by a simulator program that takes over some functions of an operating system. The simulator program according to various embodiments is capable of controlling thread spawning, preemption, operating system calls, interprocess communications, signals. Notable advantages of the invention are its capability of testing uninstrumented user applications, independence of the high-level computer language of a user application, and machine instruction level granularity. The simulator is capable of obtaining outcomes of reproducible execution sequences, reproducing faulty behavior, and providing debugging information to a user.