Free and open source software (F/OSS) communities are self-organizing, social entities that collaboratively create knowledge and innovate. Their fundamentally new approach of developing software challenges traditional principles of collaboration and learning. In contrast to well-organized and planned commercial projects, F/OSS development constitutes a continuous, iterative process of constant, incremental improvements made by various self-motivated contributors. Within such projects, organizational structures emerge that enable a large number (i.e., hundreds or even thousands) of volunteers to commit themselves to freely chosen work, yet collaboratively realize a joint enterprise. The success of F/OSS communities genuinely depends on a constant flux of new members in order to ensure the sustainability. These aspirant members must be culturally integrated and taught, in order to become expert members. This, in turn, increases complexity. Hence, these integration processes must be sophisticated, yet simple. Project coordination and new member integration, therefore, play a key role for the success of F/OSS communities. This is a challenging task, given that developers rarely meet face-to-face. New member integration takes place in online environments. It is their design and usage that are crucial for the success of such online efforts. The aim of this chapter is to discuss new member integration and learning, firstly in a theoretical manner applying a “communities of practice” perspective on F/OSS communities, and secondly by providing empirical evidence from the KDE project.