Teachers of English to Speakers of Other Languages (ESOL) in English-speaking countries are not usually required to have proficiency in another language. Teacher competency statements frequently require "an understanding of second language development," and it is assumed that a monolingual teacher can attain such understanding without having learned a second language (L2). This paper sets out to challenge such a position by establishing a theoretical framework within which to argue that teacher language learning is an important contributor to professional practice. This framework is based on research into teacher cognition, particularly that which highlights connections between teachers' lived experience and the ways in which they form their beliefs about their profession (Freeman, 2001). Using data from an Australian study, this paper shows that experiential knowledge formed by different kinds of L2 learning (formal, informal, childhood, adult, elective, or circumstantial bilingualism) forms a powerful resource underpinning ESL teachers' professional knowledge and beliefs about language teaching.