English as a second language (ESL) is taught in Australia to adult learners of mixed language backgrounds through the medium of English, and there is currently no requirement that ESL teachers speak another language. This paper reports on a study which asked what advantages there may be for ESL teachers to have proficiency in two or more languages. Data were gathered from semi-structured interviews and language biographies to explore the connection between the language backgrounds of multilingual and monolingual teachers of ESL, and their knowledge and beliefs about language learning. Theoretical constructs from the field of teacher cognition are used to argue that teachers' own language learning experience is a resource which is a powerful contributor to their conceptions of language, language use and language learning. The more multilingual the teacher, and the more varied his or her learning experiences, the richer this resource appears to be.