Preparing students for dealing with sustainability issues is a challenge in the field of education. This is a challenge because we don't know exactly what we are educating for, as there are no defined answers or outcomes to the issues; the future is unpredictable. Dealing with these issues requires crossing boundaries between people coming from different 'practices', e.g., disciplines, cultures, academia versus society, thereby making the learning and working process a challenging but critical learning experience in itself. We argue that education for sustainability should not primarily focus on student content knowledge or development of certain products or answers. It should focus on stimulating students to go through boundary-crossing learning processes critical for getting a grip on the unpredictable future. This allows students to learn to work with 'others' around the boundaries, and thereby to develop the ability to co-create new knowledge and work towards innovation or transformation for sustainable practice. Building on the boundary crossing theory and using mixed methods and interventions, this design-based study iteratively develops a boundary crossing rubric as an instrument to operationalise student learning in transdisciplinary projects into concrete student behaviour. This rubric in turn can explicate, stimulate and assess student learning and development in transdisciplinary sustainability projects.Sustainability 2019, 11, 969 2 of 20 integration of sustainable development into higher education curricula, at least in Flanders, is still scarce. Svanström et al. [10] showed in their analysis of various educational programs for sustainable development that the learning objectives varied in number and kind, but that "the integration of different perspectives" was one commonality found in all programs. Filho and colleagues [4] moreover stress that higher education should encourage more multi-stakeholder dialogue between students and (non-academic) organisations, confronting students with various aspects (e.g., economic, social, cultural or environmental) relevant for sustainable development.This requires transdisciplinary education [11] in which students work on real world wicked problems, as well as collaborate across disciplinary and institutional boundaries to work towards knowledge co-creation [12], innovation and transformation [1]. Though transdisciplinary education is sparsely emerging in higher education [3,11], most educational practices address well-structured problems that do not require multi-stakeholder interaction, probably because developing, implementing and assessing education that fosters sustainability competencies is a challenge for teachers and students [5,7]. Teachers are not educated in an inter-or transdisciplinary manner, or might not feel able to integrate sustainable development issues into their education [4,9]. Various reviews on assessment tools have also aimed to compare, evaluate or audit sustainability programs [13,14], and have shown that the actual design and activiti...