Recent New Zealand Ministry of Education publications have made it clear that mainstream subject teachers in secondary schools are expected to play an increasingly important role in the English language learning of English language learners (ELLs), rather than this being the concern of specialist English language teachers. The first step for mainstream teachers is to obtain information about their ELLs, and a number of resources have been provided for teachers to use. This paper reports on the results of a survey of the knowledge that secondary mainstream teachers have of their ELLs’ background and their level of English proficiency. The findings showed that secondary mainstream teachers’ knowledge of their ELLs varies, but nearly all of the teachers perceived that knowledge of their ELLs is important, and they reported that they would like to know more. The research has implications for secondary mainstream teachers in general, and for those who train or provide professional development for secondary mainstream teachers.