This two-stage study was undertaken to assess the extent and nature of Critical Incident Stress (CIS) amongst frontline staff in a large ambulance service in Ireland. In Stage One, 63% (112/180) of participants completed a Screening Questionnaire and the GHQ-12. In Stage Two, 27 participants, who had experienced a critical incident (CI) during the previous year completed several measures to assess PTSD symptomatology, burnout, health-related Quality of Life, and dispositional optimism. Eighty-one per cent (80/94) of the Stage One group reported that their health had been affected by a CI; 42% (44/106) were identified as 'cases' on the GHQ-12. Stage Two results indicated that 12 participants had PTSD symptoms while this entire group showed moderate levels of emotional exhaustion and depersonalization, despite experiencing high levels of personal accomplishment and optimism. The findings suggest a high prevalence of CIS among ambulance personnel in Ireland and a significant impact on overall health and wellbeing. This has important implications for the effective management of CIS and suggests an important role for occupational health and organizational psychologists in providing routine support to ambulance service staff andpossibly other emergency services personnel.