Queuing theory studies the behavior of systems subject to different working conditions, which sometimes force clients to wait for service. It has a vast applicability, since it quantifies the dilemma of many companies and institutions between efficacy (provide a good service) and efficiency (reduce the cost). However, the models do not always have a direct interpretation, reducing their helpfulness. In order to link queuing theory with management, we first explain the relevance of waiting time to the quality of service. Then we identify a number of key performance indicators, related to efficacy, efficiency and the design of the system. For different configurations, we present models that relate these indicators and show examples and applications in factories, service, logistics and health organizations.