Queueing theory is a discipline of applied mathematics that studies the behavior of lines. Queueing theory has successfully modeled throughput in a variety of industries, including within the emergency department (ED). Queueing equations model the demand for different processes within the ED, and help to factor in effects of variability on delays and service times. Utilization is a measure of the throughput of a process relative to demand, and provides a quick means of comparing the demand for certain resources. Although there have been some significant successes in applying queueing theory to EDs, the field remains underused within ED operations.