What is service discovery and why is it needed in microservices?

Service discovery enables services to find each other's network locations without hardcoding. Necessary because: instances scale dynamically, IPs change, need load balancing across instances. Patterns: client-side (clients query registry and load balance - Eureka), server-side (load balancer queries registry - Kubernetes services). Registry: Consul, etcd, ZooKeeper, DNS-based. Services register on startup, deregister on shutdown, use heartbeats. Essential infrastructure for dynamic microservices environments.