This work explains the legal relationships between licensors and licensees using the example of the popular General Public License (GPL) under German law. The focus is on contract law, copyright law, and liability. It is elaborated in detail why, despite the gratuitous nature of the open source license, it does not constitute a gift in the sense of the Civil Code. The process of contract conclusion and the extent of liability and warranty are also discussed in depth. In the copyright section, it is analyzed whether "open source use" represents an independent and separable form of use. A commentary on the parts of the GPL that are important for practice concludes the work and is also aimed at interested non-lawyers.