Case studies of the Oklahoma City Bombing, the Columbine School Shooting, and the terrorist attacks on the United States on September 11, 2001, are presented to illustrate the different crisis intervention responses and lessons learned from mass traumatic events. The authors review the theoretical and research literature on individual, large group, and emergency workers’ reactions to mass trauma followed by a discussion of the bereavement process for family and friends of victims. Next, the basic primary, secondary, and tertiary needs of victims during and after the time of crisis are reviewed and include the basic needs of survival such as water and food followed by counseling and aftercare services. Best practices are presented for meeting the crisis intervention needs of adults, children, emergency workers, and those grieving the loss of their loved ones. Best practices include providing a supportive and sympathetic response to victims during and immediately following the event, providing aftercare for children and adults, and providing mental health services to emergency workers and their families. The section on best practices is followed by the three case studies that illustrate the variety of crisis intervention responses to mass traumatic events. Lessons learned in the aftermath of the crisis response are described and include the need to develop multi-agency and large-scale disaster response plans, the need for seamless emergency communications, and the need for evacuation plans from the upper floors of high rise buildings. In closing the authors note that while it may not be possible to completely prevent terrorist acts, lessons learned from past mass traumatic events can help mute the psychological and physical trauma of victims and their families. The chapter concludes with discussion questions, simulated exercises, and a listing of Internet resources. Appendix, references