Inner Source (IS) is the use of open source software development practices and the establishment of an open source-like culture within organizations. The organization may still develop proprietary software but internally opens up its development. A steady stream of scientific literature and practitioner reports indicates the interest in this research area. However, the research area lacks a systematic assessment of known research work: No model exists that defines IS thoroughly. Various case studies provide insights into IS programs in the context of specific organizations but only few publications apply a broader perspective. To resolve this, we performed an extensive literature survey and analyzed 43 IS related publications plus additional background literature. Using qualitative data analysis methods, we developed a model of the elements that constitute IS. We present a classification framework for IS programs and projects and apply it to lay out a map of known IS endeavors. Further, we present qualitative models summarizing the benefits and challenges of IS adoption. The survey provides the first broad review of IS literature and systematic arrangement of IS research results.