Many queueing situations such as computer and telecommunication systems have the feature that customers who find all servers busy upon arrival are obliged to leave the service area and to come back to the system after a random amount of time. Then, repeated attempts for service from the pool of unsatisfied customers are superimposed on the normal stream of arrivals of first customers. The non-homogeneity caused by the flow of repeated attempts is the key to understand most analytical difficulties arising in the study of retrial queues. The pioneering papers present retrial queues as an alternative to loss models in telephony design.