The Twelve-Factor App is a methodology for building software-as-a-service applications created by Heroku co-founder Adam Wiggins in 2011. It codifies best practices for building cloud-native apps that are portable, resilient, and scalable across modern cloud infrastructure. The methodology emphasizes declarative configuration, stateless processes, and strict separation of concerns across twelve core principles. Originally designed for the PaaS era, these factors remain foundational to microservices, containerized applications, and cloud-native development in 2026—though modern extensions (15-factor, 16-factor) address AI workloads, observability, and API-first design.