Recent research shows the increasing importance of generating innovative IT solutions within information technology outsourcing (ITO) relationships. This literature review sheds further light on our current body of knowledge with regard to such innovation sourcing engagements. Based on a structured literature review and using citation network analysis methods, we (1) identify key articles and authors covering the topic of innovation sourcing, (2) analyze the development of innovation sourcing in ITO relationships over time, and (3) evaluate relevant concepts for innovation sourcing engagements in practice. Our main finding is the identification, categorization, and relevance ranking of 103 concepts related to innovation sourcing. Additionally, we identify research gaps as well as confirm that citation network analysis is a proper method for the visualization and analysis of literature in the research stream of innovation sourcing. We emphasize further research on innovation sourcing engagements and provide distinctive research directions for the research community.