Cilium is a Kubernetes-native networking, observability, and security solution built on eBPF. It provides CNI plugin functionality with overlay/native routing modes, distributed load balancing (replacing kube-proxy), cluster mesh for multi-cluster connectivity, identity-based network policies (L3-L7), service mesh with mutual authentication, and observability via Hubble (flow metrics, DNS insights). The architecture features a modular design using the Hive dependency injection framework, with components including the cilium-agent daemon, operator for CRD management, CLI tools for management/debugging, and API servers for inter-component communication. The data plane leverages eBPF/XDP for high-performance packet processing, while the control plane integrates deeply with Kubernetes through client-go, controller-runtime, and custom resource definitions.