A 28-month study was made of the delivery of emergency medical services (EMS) in disasters and large casualty-producing situations. Focus was on the organizational and human aspects of EMS in such situations rather than technical medical matters. Intensive interview, observational, documentary and statistical data were obtained in field work in 29 actual natural and technological disasters, and five potentially high mass casualty pre-planned events (e.g., the Mardi Gras celebration); additionally, in-depth studies were made of hospital and medical sector EMS preparedness in six disaster prone communities. Quantitative and qualitative analyses were made of the data by using a model which linked the pre-impact conditions of the established EMS local community system to the characteristics of the emergent EMS system.