Devices that record parameters of system performance, found in many complex systems, provide valuable investigative data. The types of devices capture data in complex systems are audio/video recorders and system state recorders, although on occasion devices intended for other purposes may provide helpful information. The value of the data that system recorders capture was evident in the investigation of a 1997 passenger train derailment. The quality of system state recorders, though relatively immune to many of the features that could degrade audio recorder data, is primarily influenced by two factors, the frequency with which the data are sampled, and the number of system parameters that are recorded. Audio recorders chronicle operator comments and other sounds heard in operating stations, and system state recorders record key system parameters, investigators may also obtain valuable data from other devices, such as security cameras. The quality of the recording components and the level of ambient noise affect the quality of audio recorder data.