[Truncated abstract] Background : Triage systems have evolved from battlefield casualty prioritisation tools to integral components of civilian emergency care systems over the last 50 years. There is significant variation in prehospital triage practices in Australia and little research has been undertaken to validate the triage systems used. There is considerable evidence to support the use of the Australasian Triage Scale (ATS) for triage in the emergency department setting and the ATS is used ubiquitously for emergency department triage in Australasia ... Conclusions : The findings of this thesis support integrating prehospital ATS allocations with emergency department triage processes. It is concluded that Paramedics apply the ATS similarly to nurses ... Allocations to ATS 1, 2 and 3 and most ATS 4 allocations by paramedics are valid when compared to nurse ATS allocations. Australasian Triage Scale category 5 is used inappropriately by paramedics and should be used rarely, if at all, by paramedics. The reliability of paramedic and nurse ATS allocations is sufficient to warrant a trial of the omission of retriage of ambulance presentations at Perth metropolitan emergency departments. However, early nursing assessment of a small proportion of ATS 3 patients may be required to ensure timely assessment for some mistriaged bone fide ATS 2 patients. Paramedic ATS allocations appear sufficiently reliable and valid to warrant a trial of their use as part of a two-tier trauma team activation system ... The implementation of standardised training between paramedics and nurses based on current Australasian College for Emergency Medicine guidelines is recommended. The implementation of paramedic triage audit, including comparison of paramedic ATS allocations with nurse ATS allocations may improve reliability between paramedics and nurses, and particularly the reliability of ATS 4 and ATS 5 allocations. Prehospital ATS allocations may prove useful in prehospital casemix analysis, the evaluation of prehospital service delivery and for prehospital research. Research opportunities include actual trials of the integration of prehospital use of ATS with emergency department triage and trauma system activation, and the evaluation of the ATS as a prehospital casemix and performance evaluation tool. Research into alternative triage tools to the ATS for use in the prehospital environment and into the impact of standardised triage training is also suggested.