The year 2015 marks the end of the UN Decade of Education for Sustainable Development (UNDESD) and the beginning of the UN Global Action Programme (GAP). UNESCO’s final report on the UNDESD (2014) acknowledges that significant achievements have been made between 2005 and 2014 in multiplying universities’ contributions to advancing sustainable development. A priority area of action for the next five years in the UN GAP will be to embrace ESD more holistically in higher education institutions (HEIs). With regard to curriculum reform, it is suggested to implement sustainable development as a “learning outcome and lifelong orientation across all fields of study” (UNESCO 2014, 127). While there is an ample account of practical initiatives to implement ESD into university curricula and study programs, it is at the same time contested that research on Higher Education for Sustainable Development (HESD) is dominated by case studies (Corcoran et al. 2004) and that with regard to learning processes “ESD remains poorly researched and weakly evidenced” (Tilbury 2011, 9). Against this backdrop, research is called for that is able to employ more comprehensive approaches by combining or integrating different research traditions and by extending the snapshot scope of short-term studies (Barth 2015, Reid & Scott 2013). Mixed-methods and longitudinal research designs have the potential to overcome the aforementioned shortcomings. However, they do not yet feature prominently in (H)ESD research (Reid & Scott 2006, Barth & Rieckmann 2013).This paper aims to contribute to the debate about the requirements, potential risks and benefits using of mixed-methods and longitudinal research designs in (H)ESD research. It introduces and discusses the longitudinal mixed-methods design of a study that is currently being implemented at Leuphana University Lüneburg in Northern Germany (2012-2016). Sustainability is a key overall education objective at Leuphana University, an example of this being an extensive module on sustainability that is delivered to all students of the first semester as a compulsory part of the curriculum, independent of their future academic specialization (Michelsen 2013). The overall objective of the study was to explore what and how students learn related to sustainable development from 1st semester to graduation, and in how far these learning processes and outcomes are influenced by the university’s curricular offers. The design of the study that this paper draws on involves three cohorts of students (commencing 2012, 2013 and 2014) as well as an external comparison group from other universities. Data is assessed several times during the course of their studies (before and after semester one, in semester four and six) by means of online questionnaires and focus groups interviews.MethodThe methodological analysis and discussion of the study design in this paper builds on some conceptual distinctions and differentiations made in the mixed methods discourse. Exemplary key questions in this respect are (Bergman 2010, Jäger-Erben et al. 2012, Johnson et al. 2007): - What objectives are pursued with mixing methods? (e.g. complementarity, extension, validation) - On which levels and with what weight are the different methods interlinked? (e.g. multi method, integration,triangulation) - What concepts does the study focus on and how do they relate to the different methods used? - How are quantitative and qualitative methods integrated? (e.g. common research questions, sample generation, data collection and analysis, interpretation and findings) - How are quantitative and qualitative methods sequenced? (e.g. preliminary study model, generalization study model, triangulation study model) In the light of these questions the paper provides a discussion of the study design to state how it increases the level of integration. For each question practical guidance and potential pitfalls will be outlined. Where appropriate, the approach of the study to realize existing research standards is critically reflected. As a result the paper contributes to the debate on quality criteria for mixed-method research.Expected OutcomesExpected outcomes exist in three different respects. Firstly, the methodological reflections on the design of the longitudinal mixed-methods study are informed and guided by a heuristic set of key questions that refer to different approaches to and traditions of mixed-methods research. This heuristic set of key questions marks an outcome itself that can be of potential value also to other researchers in the field. Secondly, reflections on a methodological level are also help to further elucidate and specify the contributions that the study can make not only in comparison with existing studies on student learning in HEIs, but also with regard to advancing more comprehensive study and research designs in the field. Thirdly, and finally, the reflective exercise and methodological discussion will hopefully stimulate more intense discussion and possibly new perspectives for comparative research on pitfalls in designing and applying a longitudinal mixed-methods approach and additional values that it can possibly add to the evidence base within the broader ESD research community.ReferencesBarth, M. & Rieckmann, M. (2013). Current trends and approaches in research in Higher Education for Sustainable Development – an international literature review from 1992-2012. Paper presented at ERSCP-EMSU 2013, Istanbul. Barth, M. (2015). Implementing sustainability in higher education: Learning in an age of transformation. London: Routledge. Bergman, M. M. (2010). On Concepts and Paradigms in Mixed Methods Research. Journal of Mixed Methods Research, 4(3), 171–175. Corcoran, P. B. et al. (2004). 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