Designing sustainable solutions for complex environmental problems calls for professionals with boundary crossing skills. The thesis developed heuristic principles for teaching and learning activities that enable environmental science students to acquire these skills. The focus was on interdisciplinary and transdisciplinary cognitive skills as a sub-set of boundary crossing skills, and on the potential contribution of conceptual models and environmental systems analysis in teaching and learning these skills. The studies that comprise the thesis were based on a literature review, analysis of courses and course material, personal experience and analysis of reflection papers written by students in authentic learning settings at Wageningen University. The thesis showed that acquiring boundary crossing skills requires learning activities that involve a combination of experience in concrete interdisciplinary or transdisciplinary projects, close interaction and debate with persons with other scientific or cultural backgrounds and interests, theory training and explicit moments of reflection. Theory input might consist of integrative environmental systems analysis methods, models and tools, but also of the theoretical and philosophical aspects related to problem oriented environmental research.