## 4 Discussion  
The results of this SSLR indicate that the amount of research on the topic of sustainability and SD in HEIs is steadily increasing. The challenges that sustainability and SD address [1], as well as HEIs' significant role regarding these challenges, are globally recognized [1; 2; 3; 6; 7; 9; 10; 11; 13]. However, taking into account the regional context, this literature review shows that most articles targeted HEIs in North America (USA and Canada), South America (Brazil), Asia-Pacific (Australia and China), Europe (Germany, Spain, Portugal, Sweden, and Belgium), and South Africa. Previous literature recognized the positive influence of internal and external HEI stakeholders on their progress towards SD [1; 3; 7; 10], yet, in this review, a lack of consensus on whether and how these stakeholders support the effective integration of SD in HEIs was identified [29; 37; 160; 162]. Our review suggested that the field of sustainability and SD in HEIs can be categorized according to the seven elements of an HEI system identified in the available literature [3; 5], i.e., institutional framework, campus operations, education, research, outreach and collaboration, SD through on-campus experiences, and assessment and reporting. The results of the final sample analysis show that the institutional framework, research, and assessment and reporting in the context of sustainability and SD in HEIs were the most heavily investigated categories during the study period, although education and research are commonly referred to as core areas and fields of action in HEIs [1; 2; 3; 6; 9; 10; 11; 13; 191]. In the category \"institutional framework,\" the articles mostly addressed organizational change to support sustainability and SD in HEIs, as well as how to apply and integrate them. The authors of these papers suggested that more in-depth investigations, empirical studies, and assessments of sustainability drivers and barriers and frameworks focusing on aspectsof change are needed [20; 22; 26; 27]. They also indicated that the stakeholders' perceptions of each barrier require further study to derive the impacts of those perceptions and to identify and test ways to overcome barriers [20; 23; 24]. Future research involving a variety of stakeholders and analyses of HEI plans, policies, and strategies was proposed to gain deeper insights into sustainability implementation, best practices, and recommendations [5; 42; 43; 44; 45; 46; 47; 56]. Communication of sustainability efforts was identified as a key to the effective management of SD implementation in HEIs [21], and SD policies were recognized as valuable tools for this communication [41]. However, a gap regarding SD policies and the aspects they address was identified. The aspects addressed in these policies were not equally emphasized, most addressed was campus sustainability, and research and education were mostly neglected [42]. A similar gap was identified when analyzing available tools, methods, frameworks and approaches [38], and HEIs' strategic plans [56]. This implies that contrary to previous literature [6], HEIs' impacts in the field of sustainability are not fully recognized [4]. As mentioned earlier, most SD policies addressed campus sustainability [42], but a significantly lower number of articles in this category indicated a lack of consensus between the practice and the literature. The articles in the category \"campus operations\" mainly addressed the EMS and its implementation process [64; 65; 66; 67; 68]. Further research, especially in the form of case studies, is needed in this category to identify and share best practices [65; 69; 70; 71; 72; 73; 74; 75]. Studies in the category \"education\" examined curricular innovations and provided reviews of SD integration and the development of sustainability-focused courses and programs, whereas articles in the category \"research\" mostly reported research on ESD. Although much literature has been published on ESD, research on how the HEIs engage with the SDGs is lacking [134].