This paper systematically reviews extant literature in open innovation (OI) on the micro-level of analysis. Empirical research in OI and previous reviews have predominantly focused on the organizational level and typically adopted firm centric perspectives. Examining the micro-level is as an important complement to this established focus and aligns with recent shifts in the literature towards the use of additional and multi-level perspectives of OI. To this end the paper identifies 104 articles on the micro-level published in top innovation journals between 2003 and 2017. Business units, projects and individuals are identified as distinct sub-levels on the micro-level and thematic categories are formed to categorize and review articles, based on their subject matter. Results show a significant amount of research on the micro-level but also imbalances in attention between the sub-levels and thematic categories. Later sections of the paper identify gaps in the literature and emphasizes particularly important areas for further development on the business unit, project and individual levels.