GNUnet is an alternative network stack for building secure, decentralized and privacy-preserving distributed applications. Our goal is to replace the old insecure Internet protocol stack. Starting from an application for secure publication of files, it has grown to include all kinds of basic protocol components and applications towards the creation of a GNU internet. This habilitation provides an overview of the GNUnet architecture, including the development process, the network architecture and the software architecture. The goal of Part 1 is to provide an overview of how the various parts of the project work together today, and to then give ideas for future directions. The text is a first attempt to provide this kind of synthesis, and in return does not go into extensive technical depth on any particular topic. Part 2 then gives selected technical details based on eight publications covering many of the core components. This is a harsh selection; on the GNUnet website there are more than 50 published research papers and theses related to GNUnet, providing extensive and in-depth documentation. Finally, Part 3 gives an overview of current plans and future work.