Immutable infrastructure is a modern infrastructure paradigm where servers and components are replaced entirely rather than modified in-place after initial deployment. This approach eliminates configuration drift, increases deployment reliability, and ensures consistency across environments by treating infrastructure as disposable artifacts that are versioned, tested, and rebuilt from scratch for every change. The "cattle not pets" philosophy and phoenix server pattern form the foundation of this methodology. Immutable infrastructure aligns naturally with containerization, Infrastructure as Code (IaC), and cloud-native architectures, enabling automated, reproducible, and predictable deployments at scale.