UK emergency departments (EDs) are high-pressure environments focused on delivering care in the most efficient way to patients with a range of health problems. For many people EDs are the front door of the NHS and are a focus of significant media and political interest. People who attend EDs are often anxious and a main element of their concern is waiting time for treatment. In UK EDs the four-hour target is a main NHS target and a cornerstone of evaluating ED performance. There is ongoing debate about whether spending additional time in EDs affects patient care and outcomes, with some research showing increased mortality associated with longer stays and some showing no effect on mortality. Evidence suggests that patients are spending longer in UK EDs and it is possible that those who remain longer than four hours could have worse outcomes. This article identifies the effects of prolonged ED length of stay through a systematic literature review of data published since implementation of the four-hour target to measure the relationship between breaching the target and morbidity and mortality.