Context: Inner source software development (ISSD) has been viewed as an alternative approach in which organisations adopt open source software development (OSSD) practices and exploit its benefits internally.Objective: In this paper, we aim to provide an extensive review of current research on ISSD and to establish a research agenda on this domain.Method: The review is primarily performed using a systematic literature review protocol.Results: We identified, critically evaluated and integrated the findings of 37 primary studies, describing 25 empirical research papers, 10 frameworks/methods, models and tools to support the implementation of inner source, as well as a set of benefits and challenges associated with ISSD.Conclusion: This study presents four main contributions. First, the study provides an in-depth review of ISSD to date, i.e. the evolution of research across inner source, contributions of existing research developments, and theories, models and frameworks used to study inner source. Second, our review applies the OSSD approach framework as the lens to analyse ISSD. Third, the review updates the key challenges associated with ISSD from a management perspective. The final contribution is the establishment of a research agenda to advance knowledge on ISSD. (C) 2020 Elsevier Inc. All rights reserved.