This paper describes the methodology of a large emergency medical services (EMS) data linkage research project currently under way in the statewide EMS system of New South Wales, Australia. The paper is intended to provide the reader with an understanding of how linkage techniques can be used to facilitate EMS research. This project, the Australian Prehospital Outcomes Study of Longitudinal Epidemiology (APOStLE) Project, links data from six statewide sources (computer-assisted dispatch, EMS patient health care reports, emergency department data, inpatient data, and two death registries) to enable researchers to examine the patient's entire journey through the health care system, from the emergency 0-0-0 call to the emergency department and inpatient setting, through to discharge or death, for approximately 2.6 million patients transported by the Ambulance Service of New South Wales to emergency departments between June 2006 and July 2009. Manual, deterministic, and probabilistic data linkages are described, and potential applications of linked data in EMS research are outlined.