New Zealand Emergency Department (ED) attendances are growing at 4-5% per year. Substance use (SU) and mental health (MH) issues constitute a considerable, but poorly understood or documented, burden on Emergency Department services. This research will involve a clinical data audit; a series of analyses of six years of data from the Wellington Hospital Emergency Department Information System (EDIS). To date in New Zealand, little research attention has been directed to the prevalence of substance use and mental health presentations in the ED, or their impact on ED functioning, their outcome, and the health system costs associated. This study will examine in detail the prevalence and correlates of SU and MH in the Emergency Department, analyse changes over time, estimate costs, and identify opportunities for developing targeted interventions to reduce ED use and provide screening and early intervention to those who are identified as at risk.