A simulator and/or simulation process operates by receiving a message from a system; comparing the received message to information stored in a response file used to simulate system response; and simulating a response to the system by outputting a response stored in association with a stored message matching the received message. The response file includes at least one message, a message marker associated with each message, at least one response associated with each message, and an end-of-response marker associated with each response. At least two responses are stored in association with a message, and the at least two responses are sequentially output in response to sequential receipt of the message. As such, the simulator can reply to any message by looking up its associated response in the response file. If more than one associated response appears in the response file, receipt of identical messages will illicit sequential and differing responses. Thereby, an effective simulator and/or simulation process can easily and simply be developed to thereby enable development of various components of a system in an environment independent of other components of the system.