This paper examines a language teacher education professional development programme in New Zealand that draws on the 2007 New Zealand Curriculum. At the heart of the Learning Languages area in the curriculum is communicative competence, with the understanding that communication involves language knowledge and cultural knowledge. The New Zealand Ministry of Education expects that schools will be able to offer all Years 7–10 students the opportunity to learn an additional language in order for them to participate effectively in multicultural settings, both in New Zealand and internationally. To deliver this, language teachers and generalist teachers are being encouraged to undertake professional development. This paper reports on a research evaluation of a Ministry-sponsored language teacher professional development programme. The findings reveal success in increasing teacher understanding of how to develop learners’ language knowledge, because this part of the programme was underpinned by a deep principled knowledge base, and teachers had opportunities to acquire knowledge and participate in a language teaching community. However, teacher understanding of how to increase learners’ cultural knowledge was less successful, because of a lack of a principled knowledge base of intercultural language teaching. We argue that effective professional development programmes need both to be based on deep principled knowledge and to offer learning that involves acquisition and participation.