This document provides an introduction to queuing models and queueing theory. It discusses how queueing theory is the mathematical study of waiting lines and models customer service systems. Examples are given of where queues commonly occur, such as hospitals, stores, and offices. The key aspects of queuing models are described as analyzing how customers receive service when there is variability in arrival times and service durations. Common queuing models are outlined, including single-server and multi-server models, and priority queuing is discussed. The wide applicability of queuing theory to optimize systems in many fields like healthcare, transportation and computing is also mentioned.