In recent years, the service-learning (SL) methodology has experienced an unprecedented boom in the countries within our political and cultural sphere, creating a challenging terrain for researchers interested in studying its impact, measuring its outcomes, and even effectively developing its implementation. This pedagogy, which integrates academic learning with community service, fosters deep and meaningful educational experiences for those involved, surpassing the cognitive-social expectations of more conventional educational strategies.

The inherited university model from the past (20th century and earlier), sometimes referred to as the "ivory tower," has recently faced significant criticism from both civil society and the academic sphere due to its perceived detachment from social reality. In our immediate context, this underlying issue influenced efforts to adopt the European Higher Education Area (EHEA), the seed of more comprehensive views on learning and competency development (Santos Rego, Mella-Núñez et al., 2020).

We are talking about a paradigm shift, as the central idea promoting it is that knowledge, dynamically linked to human activity, should ideally resonate with the notion of (social) change. Therefore, it is not surprising that SL has become a significant strategy for bridging the university-community gap. However, it seems clear that its scope and effectiveness will depend largely on the quality of experiences, and it is the responsibility of the research community to discover the best strategies for these purposes.

In summary, we could discuss two predominant approaches in research on SL (Bringle, 2003): on the one hand, the comparison of subjects where the methodology is applied with those using more conventional strategies, and on the other hand, the evaluation of how different course configurations impact better or worse outcomes.

It is evident that when delving into the study of SL implementation, we encounter a rich diversity of experiences, interventions, and contexts reflecting the adaptability of the methodology. However, it is precisely in this variety where difficulties arise in evaluating the quality of courses accurately and coherently. Thus, energizing a homogeneous method to study them becomes a challenge of great relevance.

It should not be surprising, therefore, that due to this added difficulty, many studies point out the lack of research on the conceptual and operational quality of the methodology itself. Most analyze the results obtained by participants, ignoring other issues related to the form of participation (Imperial et al., 2007). Sufficient reason to consider quality evaluation a promising research line, especially if we want to ensure that SL courses achieve their goals and intensify benefits, naturally including the necessary emphasis on civic engagement.

Historically, we attribute to Sigmon (1979) the merit of establishing the first quality principles in Service-Learning. Since then, we have witnessed a sufficiently constructive dialogue that would impact future SL courses and the deliberative axis around what are, or should be, best practices in using the methodology. Despite the emergence of multiple classifications to elucidate quality programs, what matters is to show dimensions with a real impact on results.

However, the study we have conducted primarily aims to analyze the most relevant quality criteria in Service-Learning. To achieve this, we carried out a systematic literature review, specifically focusing on understanding how criteria affected real experiences. The goal is to illuminate 'key factors' that can contribute to good methodology practices, helping - in the process - to better understand the positive impact of SL in those circumstances.

This paper is framed in the Research Projects: “Service-Learning (SL) and employability of university graduates in Spain: competences for employment” (EDU2017-82629-R) and “The impact of the university in the community through service-learning projects. A study focused on reciprocity (SL)” (PID2021-122827OB-I00).

A comprehensive global documentary review was conducted using databases such as Scopus, WoS, and ProQuest to shed light on the characteristics that define successful SL on the international stage. The search process combined keywords related to the quality elements that should be present in SL courses. To determine these quality dimensions, we relied on the most relevant best practice classifications in the field of SL, taking note of their frequency in the literature. Among them, we highlighted those of Imperial et al. (2007), Honnet and Poulsen (1989), and Howard (2001). Additionally, we considered the quality standards in the practice of service-learning in K-12 education, developed by the National Youth Leadership Council (2008). Based on these and other documents, we identified the following quality dimensions: duration and intensity, academic rigor, meaningful service, reciprocity, role of faculty and students, and reflection. We included books and articles published in both English and Spanish. Duplicate documents found in different databases were excluded. Finally, we eliminated those documents that, while relevant to the general theme, lacked sufficient relevance to the central topic of the research. In the stage of documentary classification, we distinguished two types of studies. Firstly, those that appeal, with theoretical arguments, to the importance of a series of course characteristics as indicators of thpreir success. And secondly, those that seek to scrutinize, empirically, the impact that such characteristics can have on those who participate in the experiences.

Firstly, the most effective programs are those with a duration between 20 and 40 hours (Dahan, 2016). Regarding academic rigor, it is necessary to define clear objectives that are closely related to the academic curriculum, as failing to do so is associated with a negative impact (Celio et al., 2011; Reames et al., 2020). The significance of the service is positively linked to student motivation and academic improvements (Billig et al., 2005; Moely and Ilustre, 2014). On the other hand, student autonomy is related to greater commitment and achievements, while allowing students to participate in decision-making is associated with the development of self-concept and cross-cutting competencies (Lambright and Lu, 2009). Furthermore, reciprocity and reflection criteria seem to be the most important according to the consulted literature. Regarding reciprocity, actively collaborating with recipients in long-term experiences has been associated with better outcomes for all involved parties (Bailis, 2000; Miron and Moely, 2006). Regarding reflection, a noteworthy finding is the connection between reflection in and/or with the community and more successful courses; directing these sessions towards a variety of themes throughout the entire process (before, during, and after courses) (Lorenzo et al., 2021). In conclusion, a crucial aspect in course management is, as expected, the study of quality elements, both in the theoretical and empirical realms. We have sought to provide a more comprehensive and diverse understanding of these elements, aiming for their better comprehension while keeping in mind the adaptation to the specific context of each program. We are convinced that emphasizing the definition of quality criteria for Service-Learning can become a catalyst for its effectiveness, potentially influencing the paths of future implementations within the university system. However, it is essential to ensure that these paths have the necessary support from civil society in general and communities in particular.

Bailis, L. (2000). Taking service-learning to the next level: Emerging lessons from the national community development program. National Society for Experiential Education. Billig, S. H., Root, S., & Jesse, D. (2005). The relationship between the quality indicators of service-learning and student outcomes, testing professional wisdom. In S. Root, J. Callahan & S. H. Billig (Eds.), Improving service-learning practice: Research on models to enhance impact (pp. 97-115). Information Age Publishing. Bringle, R. G. (2003). Enhancing theory-based research on service-learning. In J. Eyler & S. H. Billig (Eds.), Deconstructing service-learning: Research exploring context, participation, and impacts (pp. 3-22). Information Age Publishing. Celio, C. I., Durlak, J. A., & Dymnicki, A. B. (2011). A Meta-analysis of the Impact of Service-Learning on students. Journal of Experiential Education, 34(2), 164-181. https://doi.org/10.5193/jee34.2.164 Dahan, T. (2016). Revisiting pedagogical variations in service-learning and student outcomes. International Journal of Research on Service-Learning and Community Engagement, 4(1), 3-15. https://doi.org/10.37333/001c.29586 Honnett, E. P., & Poulsen, S. J. (1989). Principals of good practice for combining service and learning. https://digitalcommons.unomaha.edu/cgi/viewcontent.cgi?article=1001&context=slceguides Howard, J. (2001). Service-learning course design workbook. OCSL Press. Imperial, M. T., Perry, J. L., & Katula, M. C. (2007). Incorporating service learning into public affairs programs: Lessons from the literature. Journal of Public Affairs Education, 13(2), 243-264. https://doi.org/10.1080/15236803.2007.12001478 Lambright, K. T., & Lu, Y. (2009). What impacts the learning in service learning? An examination of project structure and student characteristics. Journal of Public Affairs Education, 15(4), 425-444. https://doi.org/10.1080/15236803.2009.12001570 Lorenzo, M., Sáez-Gambín, D., Ferraces Otero, M. J., & Varela, C. (2021). Reflection and Quality Assessment in Service-Learning Projects. When, with whom, and why. Frontiers in education, 5, 1-14. https://doi.org/10.3389/feduc.2020.605099 Miron, D., & Moely, B. E. (2006). Community agency voice and benefit in service-learning. Michigan Journal of Community Service Learning, 12(2), 27-37. Moely, B. E., & Ilustre, V. (2014). The Impact of Service-Learning Course Characteristics on University Students' Learning Outcomes. Michigan Journal of Community Service Learning, 21(1), 5-16. http://files.eric.ed.gov/fulltext/EJ1116526.pdf National Youth Leadership Council. (2008). K-12 Service-Learning Standards for Quality Practice. https://nylc.org/k-12-standards/ Reames, T. G., Blackmar, J. M., & Pierce, J. C. (2020). Teaching the three E’s of sustainability through Service‐Learning in a professional program. New Directions for Teaching and Learning, 2020(161), 73-82. https://doi.org/10.1002/tl.20374 Santos Rego, M. A., Mella Núñez, Í., & Sotelino Losada, A. (2020). Movilidad y TIC en aprendizaje-servicio: perspectivas para una sociedad global y tecnológica. RIED, 23(1) 67-84. http://dx.doi.org/10.5944/ried.23.1.24180 Sigmon, R. (1979). Service-learning: Three principles. Synergist, 8(1), 9-11. https://nsee.memberclicks.net/assets/docs/KnowledgeCenter/IntegratingExpEduc/BooksReports/55.%20service%20learning%20three%20principles.pdf