What is Queuing theory? Queueing theory is the mathematical study of waiting lines, or queues. Queueing theory is considered a branch of operations research because the results are often used when making business decisions about the resources needed to provide service. The theory enables mathematical analysis of several related processes, including arriving at the end of the queue, waiting in the queue (essentially a storage process), and being served at the front of the queue. The theory permits the derivation and calculation of several performance measures including the average waiting time in the queue or the system, the expected number waiting or receiving service, and the probability of encountering the system in certain states, such as empty, full, having an available server or having to wait a certain time to be served.