This document outlines Twitter's approach to open source software. It discusses Twitter's open source principles of using open source by default, defining proprietary "secret sauce," defaulting to permissive licenses and GitHub, acquiring and open sourcing software when possible, and paying it forward through support of other open source projects. It also describes Twitter's efforts to scale its open source program through automated processes for quality checks, sourcing contributors for hiring, and reporting on open source metrics. The goal is to maximize adoption, participation, and contribution to Twitter's open source projects.