Mainstream teachers in New Zealand secondary schools are expected to be able to support learners from linguistically diverse backgrounds, providing both content instruction and also language support that enables them to access curriculum content. The New Zealand Ministry of Education has produced resources designed to enable mainstream teachers to bring a language focus to their teaching in order to provide support for their English language learners (ELLs). This study investigated mainstream teachers’ knowledge of second language acquisition and their knowledge and use of teaching resources and strategies for supporting ELLs in the mainstream. The eighteen teachers who participated in the study felt that they needed to know more about second language acquisition. They were also not fully aware of, and were not using, the range of resources and strategies available. These findings have implications for the professional development of New Zealand secondary mainstream teachers, and for the education of ELLs.