Workplace stress is an insidious, pervasive, and increasing problem that contributes to psychological, psychosocial, and physiological declines to workers' health. This thesis focuses on the effect of emotional intelligence (EI) as a moderator of workplace stress, measured using biomarkers. EI is beneficial to workers because individuals with higher EI have stronger coping abilities, engender more trust, and perform cognitive tasks with less effort, resulting in greater life satisfaction and wellbeing. For these reasons, I argue that EI reduces workplace stress. However, research on EI and stress has long been hindered by subjective measures of both EI and stress. Physiological evidence that EI or EI training moderates the effects of work stress is lacking, and thus I investigate the relationship between EI, measured by the ability-based MSCEIT (Mayer, Salovey, & Caruso, 2002), and physiological stress, measured using salivary cortisol, in two contexts-in a lab using university students, and in the field with Special Forces military personnel.A common source of workplace stress is incivility. In the two lab studies, I assess the moderating effect of EI on participants who experienced stress during online interactions with supportive or uncivil supervisors (Study 1: N= 328,) and supportive, neutral, or uncivil supervisors (Study 2: N= 350). A 2x2x2 sampling model was developed in which subjects completed a business-like task with support or incivility from an online supervisor. EI and stress were recorded using a Mayer-Salovey-Caruso EI ability test and salivary cortisol.Based on COR theory, I hypothesised that EI associates with reduced stress and increased trust. Participants with high EI had a lower change in cortisol (i.e., less stress) in response to cyber-mediated incivility from supervisors than participants with low EI did. Individuals treated in a supportive, civil manner demonstrated significant reductions in cortisol when supervisors supported them. These results corroborate the proposition that individuals with high EI better moderate stress responses to workplace incivility than low EI individuals do.Using field studies, I tested whether EI training reduces physiological stress and increases cognitive function and behavioural performance in a real-world, extremely physical and emotionally high-risk/high-stress work environment. I expanded investigation to the effectiveness of a tailored EI training program. Research conducted in the Australian Special Forces included two studies (Study 3a: N= 35, Study 3b: N= 43), during which I examined the efficacy of EI training in managing stress during real military training exercises using salivary cortisol and Immunoglobin A (IgA) as objective indicators of stress and burnout in a practice-based, Commando population. Cognitive performance under stress was also3 measured using behavioural performance activities. Soldiers were tested before, during, and after standard training protocols of rappelling, shoot-no-shoot, self-care under fire, urban precision strike, ...