With the rapid adoption of cloud computing, remote work, and Internet of Things (IoT) devices, traditional perimeter-based security models are no longer sufficient to safeguard enterprise networks. Zero Trust Architecture (ZTA) introduces a modern security paradigm based on the principle of “never trust, always verify,” enforcing continuous authentication, micro-segmentation, and strict access controls. This paper provides a comprehensive survey of ZTA, including its core principles, architectural components, deployment models, and real-world applications. We critically analyze existing frameworks such as the NIST Zero Trust model and Google BeyondCorp, evaluating their strengths and weaknesses against evolving cyber threats. The paper also highlights implementation challenges and proposes future research directions, including AI-driven access control, blockchain-based identity validation, and ZTA integration for 5G and IoT ecosystems.