This chapter focuses on operator-related antecedents to explain the effects of operator factors. The major ways that physiological antecedents can degrade operator performance and lead to error will be reviewed and data needed to determine if a relationship between the two exists suggested. Physiological antecedents can temporarily or permanently degrade operator performance by leading to either general or specific impairment. If, operators engaged in personal activities that led to their fatigue, then the company would not be considered the source of the fatigue and the antecedent to error. Behavioural antecedents to error include fatigue and stress, originating from the operator's personal experiences or from company actions. Because fatigue causes impairment, the effects of withdrawal from sustained use of cocaine-effects that include mood alteration in addition to sleep disruption-can create antecedents to error.