The rapid propagation of the International Baccalaureate (IB) in the East and Southeast Asian region, together with the continuing globalisation of education, has generated implementational opportunities and challenges for teachers within the IB network of schools. Based on semi-structured interviews with teachers and school leaders across 15 IB Diploma Programme (DP) schools within the three milieus of Hong Kong, Taiwan and Singapore, the study explored how DP's expansion fuelled ambiguities and inconsistencies in programme policy, which impacted on professional cultures and assessments. The analysis highlights the overreliance on the voluntary contribution of teachers and external examiners in supporting professional development and assessments as well as the connection of assessments to anxiety and dissonance in teachers. Copyright ©2023 Routledge.