Priority queues are seen in a variety of real-world service applications such as theme parks, call centers, order deliveries, and government and healthcare services. Priority queues are often believed to improve efficiency, increasing both service providers’ revenue and social welfare. However, in the face of self-interested customers, queue managers must carefully take into account how customers self-select into priorities and how their queue-joining decisions are affected. This chapter reviews the theoretical literature on priority queues of self-interested customers, with a focus on pay-for-priority schemes.