## 5 Conclusions  
In this paper, an SSLR was conducted on sustainability and SD in HEIs. The focus was placed on peer-reviewed scientific journal articles. No time boundaries were set and the literature search was conducted in March and April 2020. The final literature sample consisted of 190 articles. The aim was to identify the main research theme(s) addressed and research gaps identified in the available literature. The growing number of publications illustrates the significance of the topic. However, the analysis of these publications also reveals the inconsistencies that need to be addressed by HEIs in future research and in practice. Although the number of publications on the topic is steadily increasing, the contrasts regarding the regional context and regarding areas of action addressed by HEIs show the lack of systematic approach when integrating and implementing sustainability and SD in HEIs. The publications mostly targeted HEIs in North and South America, Asia-Pacific, Europe, and South Africa, and other regions were underrepresented. The categorization of the final literature sample according to the seven elements of an HEI system, i.e., institutional framework, campus operations, education, research, outreach and collaboration, SD through on-campus experiences, and assessment and reporting, also showed the underrepresented areas that need further investigation. Campus sustainability, education, outreach and collaboration, and SD through on-campus experiences were underrepresented among the investigated categories. This again implies a lack of systematic as well as holistic approaches when integrating and implementing sustainability and SD in HEIs. In the SD policies of HEIs not all aspects were equally emphasized, which once more implies that more research is needed on systematic and holistic approaches of sustainability and SD integration and implementation in HEIs. Furthermore, fewer publications explicitly focused on the integration of SDGs in HEIs. The lack of consensus in the available literature with regard to the role and effects that stakeholders have in the integration of sustainability and SD in HEIs also deserves further examination. One of the major research gaps identified is a lack of indicators to measure the sustainability performance of HEIs. This lack of knowledge represents a clear barrier to SR in HEIs. Overall, our review enabled us to identify a need for more research in the form of case studies, projects, approaches, and methods on the topic of sustainability and SD in HEIs. Such research facilitates the identification of best practices. Moreover, further information about and examples of less successful cases are needed to be able to derive lessons from these examples. This paper represents a comprehensive overview of the literature on sustainability and SD in the context of HEIs for a broad understanding of the sustainability and SD in HEIs. The identified research gaps can serve as a starting point for a future research agenda. From a practitioner's perspective, the results of this paper may be helpful for HEIs on their path towards redirecting their actions and addressing the sustainability and SD challenges. Without systematic, holistic, and whole-institution approaches to implementing and integrating sustainability and SD in HEIs, involving all areas of action and all stakeholders, the struggles HEIs are facing with regard to fulfilling their role in society and addressing sustainability and SD challenges will remain. Furthermore, without understanding and evaluating their sustainability and SD contributions, HEIs will not be able to address the emerging areas of action in their own organizations. Conceptualization, A.O. and B.M.Z. ; methodology, A.O. ; validation, A.O. and B.M.Z. ; formal analysis, A.O. ; investigation, A.O. ; resources, A.O. ; writing--original draft preparation, A.O. ; writing--review and editing, A.O. and B.M.Z. ; visualization, A.O. and B.M.Z., supervision, B.M.Z. Both authors have read and agreed to the published version of the manuscript.