## 2 Materials and Methods  
The first step of this SSLR was the selection of the keywords. The keywords were selected by building on the relevance of the topic of sustainability and SD in the context of HEIs and the research questions provided in the Introduction section. The aim for the keywords included was to comprehensively cover the topic and research questions, taking into account the complexity of sustainability and SD and the fact that it affects all areas and activities in HEIs [2, 13]. Therefore, the keywords were defined in a broad sense to include environmental, social, and economic aspects of sustainability and SD (sustainab *, sustainab * development) in all areas and activities of HEIs (higher education institution *) [4, 9, 15]. The literature search process for this SSLR was conducted using search strings generated from the predefined keywords and the Boolean search operator shown in Table 1.  
The PRISMA flow diagram shown in Figure 1 depicts the research strategy applied in this work. The literature was identified using Scopus, the largest citation database of peer-reviewed literature. To ensure the accessibility of the literature, the language was limited to English, and the publication stage was set as final. In this work, we placed a focus on peer-reviewed scientific journal articles, since these were considered to be the most useful [15]; therefore, the source types for the search were limited to journals, and the document types were limited to articles, reviews, and conference papers. To include high-quality and up-to-date results, journals without impact factors were excluded [16]. The initial sample of articles was extracted from 200 journals. This list was reduced to 63 journals by taking into account the majority of published articles and different journal rankings, namely, the h-index, CiteScore, SCImago Journal Ranking (SJR), Source Normalized Index per Paper (SNIP), and the Journal Impact Factor (JIF). All 63 journals are indexed in the Web of Science. The full list of included journals can be found in Appendix A. To ensure that the concepts of sustainability and SD in HEIs were cited in the articles, the keywords in the documents were reviewed. For example, an additional bias resulted from the fact that one of the main publishing journals on this topic is named _Sustainability_. If the paper addressed any aspect of sustainability or SD in the context of HEIs, it was included in the final sample. The full list of revised and accepted keywords for each search string can be found in Appendix B. No time boundaries were set, and the literature search was conducted in March and April 2020. These search parameters were applied to search strings and, after the duplicates were excluded, a sample set of 356 articles was derived. The last step was to screen the titles, abstracts, and/or keywords to ensure the definitional fit to the purpose of this work and to assess the quality of articles. After screening, the articles were analyzed using Field-Weighted Citation Impact (FWCI). FWCI indicates how frequently one article is cited compared to similar articles [18]. If an article has an FWCI value greater than 1.00, that article is cited more frequently than expected compared to the global average. Therefore, all articles with an FWCI above 1.00 were included in the data sample, yielding a final sample of 190 articles. An analysis of the available literature [3; 5] indicated that the HEI system consisted of seven elements, i.e., institutional framework, campus operations, education, research, outreach and collaboration, SD through on-campus experiences, and assessment and reporting. These seven elements are represented in Table 2, along with the descriptions and examples. The information represented in Table 2 was used to categorize the final literature sample and to assign each article to one element of the HEI system. In the following, these seven elements of the HEI system are referred to as the categories of the final literature sample.