Learner diversity is a common feature of the adult English as a second language (ESL) classroom and can manifest itself in many ways including different ethnic backgrounds, first languages (L1), educational experiences, cultural values, and current English proficiency. There is, however, limited empirical research examining teachers' principles and practices relating to learner diversity in the classroom, a phenomenon which poses a challenge for novice ESL teachers. To address this gap, this study examined how experienced ESL teachers understood their pedagogic principles and practices when addressing learner diversity in their classrooms. The research recruited ESL teachers within the Adult Migrant English Program (AMEP), a large-scale and extremely important program that provides English training to eligible migrants and refugees in Australia. A qualitative case study design was employed recruiting two experienced teachers teaching different level classes within the AMEP. Using a combination of semi-structured interviews and classroom observations, four significant aspects of diversity across the learner groups were identified, only one of which -- English proficiency level -- was common to both groups. While the participants did share some principles relating to this common aspect of diversity, they also held several different principles. Notably, the study found that the participants' practices were not aligned with any particular teaching method but instead, were determined by the specific nature of their teaching context and the pedagogic principles applicable to that context.