This chapter introduces the reader to what it means for a large language model to be open source, all the aspects involved, the community consensus and stories of false claims. It gives insights on how to recognise when an LLM is truly open and how to be wary of vague marketing. We’ll explore notorious examples of modern LLMs that are open in some way or another, highlighting the level of openness each of them is currently at. The chapter closes with popular public repositories and archives to use to navigate the open source landscape of large language models.