Quality of Service (QoS) is a set of networking technologies and mechanisms that manage bandwidth, prioritize traffic, and ensure predictable network behavior by controlling packet forwarding treatment across network devices. Originally developed to support real-time applications like voice and video over shared IP networks, QoS enables network administrators to classify, mark, queue, police, and shape traffic based on application requirements. The fundamental insight: not all traffic is equal — voice calls cannot tolerate delay or jitter that file transfers can absorb — so QoS provides the tools to enforce priority hierarchies, prevent congestion, and guarantee service levels across converged networks where data, voice, and video compete for the same bandwidth.