Viper is a mature, widely-used Go configuration library that provides a unified API for reading configuration from multiple sources: files (YAML, JSON, TOML, dotenv), environment variables, command-line flags (via pflag), remote key-value stores (etcd, Consul, Firestore, NATS), and explicit overrides. It implements a priority-based merging system where later sources override earlier ones. The library offers nested key access with dot notation, aliasing, live file watching, and unmarshaling to structs via mapstructure. The architecture is a single Viper struct with internal maps for each source, plus a pluggable encoding layer and a finder interface for config file discovery. It is designed as a library to be embedded in Go applications, not as a standalone service.