Curriculum integration is a feature of many new curricula that have emerged in different countries since around the turn of the millennium. It focuses on removing the boundaries between traditional subject specialisms, to enable more holistic and 'joined-up' learning opportunities. This study draws on the experiences of a group of primary and secondary teachers in Wales, engaged in creating a framework for an integrated curriculum for expressive arts. Whilst the teachers are united in their ambition for establishing a curriculum that gives greater status to the arts, curriculum integration presents significant challenges, notably in how subject knowledge is understood and presented within an integrated curriculum. The teachers take different approaches to curriculum integration, with primary teachers favouring a "transdisciplinary" approach, with child-led learning and themes taking precedence, and secondary teachers opting for "multidisciplinary" approaches, where the themes are organising devices but where subjects take priority. Differing practices suggest differing conceptions of subject knowledge and mastery within an integrated curriculum. Drawing, in particular, on Bernstein's concepts relating to knowledge discourses, this paper suggests that the danger of an integrated curriculum is weakened disciplinary knowledge. Whilst this paper relates to the arts, the messages about curriculum integration might be applied more widely.