That the value of interdisciplinarity in sustainability education is largely taken for granted reflects widely held views about the inherent complexity and uncertainty of sustainability issues and the need for holistic responses to them. Although there is an extensive literature on interdisciplinarity, there has been relatively little investigation of the ways in which teachers and learners deal with and respond to this central aspect of sustainability education. This paper presents data from two case studies of masters course in sustainability education, one in an English university and one in a Chinese university, and examines some of the contextual factors that support or limit their attempts to go beyond the bounded ways of thinking associated with more traditional academic disciplines. While some learners respond enthusiastically to interdisciplinarity, others experience forms of "cognitive dissonance". Interdisciplinarity requires individual teachers to operate as members of teams and to find ways of promoting dialogue between different perspectives, discourses and methods of approaching problems. The key to interdisciplinarity is an open attitude, a willingness to learn and an ability to engage with different ways of thinking about issues we face in common. (Contains 4 notes.)