Many of today's most innovative products and solutions are developed on the basis of free and open source software (FOSS). Most of us can no longer imagine the world of software engineering without open source operating systems, databases, application servers, Web servers, frameworks, and tools. Brands such as Linux, MySQL, Apache, and Eclipse have shaped product and service development. They facilitate competition and open markets as well as innovation to meet new challenges. De facto FOSS standards such as Eclipse and Corba simplify the integration of products, whether they're all from one company or from multiple suppliers. IEEE Software has assembled this theme section to provide a brief yet practical overview of where FOSS is heading.