Queueing Theory analyzes waiting lines and service systems using mathematical models. A queueing system consists of: arrival process (often Poisson with rate λ), service process (often exponential with rate μ), number of servers c, queue capacity, and queue discipline (FIFO, LIFO, priority). Kendall notation A/B/c/K/N/D describes systems where A is arrival distribution, B is service distribution, c is servers, K is capacity, N is population size, D is discipline. Key measures: average number in system (L), average waiting time (W), utilization (ρ = λ/cμ). Little's Law: L = λW. Models include M/M/1, M/M/c, M/G/1, and queueing networks. Applications: call centers, healthcare, telecommunications, and manufacturing.