Docker Compose is a tool for defining and running multi-container Docker applications using a single YAML configuration file. Originally written in Python (v1) and now rewritten in Go (v2), it simplifies orchestrating complex applications by managing service definitions, networking, volumes, and dependencies through declarative configuration rather than imperative commands. Docker Compose is essential for local development environments, integration testing, and single-host production deployments where full orchestration platforms like Kubernetes would be overkill. One critical insight: while Compose handles service startup order through depends_on, it does not wait for services to be ready—only for containers to start. This is why health checks and service_healthy conditions are crucial for reliable multi-container orchestration.