Emergency medical services (EMS) systems, and prehospital care are difficult to evaluate. Accordingly, the true efficacy and value of such systems are difficult to determine. The multitude of variations and combinations of involved factors makes standardisation and comparison difficult, and universal indicators are hard to develop. Various attempts have been made to determine valid indicators of effectiveness, but there has been little success. Prehospital care has been seen by some as a single entity. As a result, experience from well resourced first world trauma centres has been taken, by many, to be applicable to all prehospital situations. This article attempts to assist in the development of valid EMS indicators of performance and effectiveness by categorising prehospital scenarios into a classification reflecting the reality of their conditions of practice.