The effects of unfavorable work environmental conditions can be studied using psychological performance measures and subjective ratings of mood and symptoms. In the present article the rationale for such investigations is discussed, and the computerized performance evaluation system developed at the Swedish National Institute of Occupational Health is presented. The background and the research experience leading to the system's development are described, and references to the numerous empirical studies demonstrating its applicability to different research objectives are given. Also included is information regarding the technical requirements for running the SPES tests.