Over the last decades, the concept of patient flow has received an increased amount of attention. Healthcare professionals have become aware that in order to analyze the performance of a single healthcare facility, its relationship with other healthcare facilities should also be taken into account. A natural choice for analysis of networks of healthcare facilities is queuing theory. With a queuing network a fast and flexible analysis is provided that discovers bottlenecks and allows for the evaluation of alternative setups of the network. In this chapter we describe how queuing theory, and networks of queues in particular, can be invoked to model, study, analyze and solve healthcare problems. We describe important theoretical queuing results, give a review of the literature on the topic, discuss in detail two examples of how a healthcare problem is analyzed using a queuing network, and suggest directions for future research.