Unwell patients in the ED requiring inpatient admission must negotiate the interface between the ED and inpatient wards. Despite its importance and scale, this ED-inpatient interface (EDii) is poorly characterised. The aim of this paper is to clearly define the EDii and to describe its importance to (i) the patient: delays to admission and errors in communication across the EDii can increase adverse outcomes; (ii) the hospital: poor EDii function reduces hospital efficiency and effectiveness; and (iii) the healthcare system: half of all hospital inpatient admissions occur via the EDii and so EDii affects system-wide performance. The EDii can be defined as the dynamic, transitional phase of patient care in which responsibility for, and delivery of care, is shared between ED and inpatient hospital services. The EDii is characterised by a complex interplay of patient, hospital and system factors. A clear definition of the EDii and an understanding of its importance will assist future research and interventions to improve patient outcomes.