The chapter presents an overview of current and recent research concerning the phonetics and phonology of adult second language (L2) learners in classroom settings, summarizing key findings and suggesting future research directions. In this context, classroom learners are defined as college-age individuals who are immersed in their first language (L1) and whose L2 learning is limited to the instructional environment of college or university classes. The survey deliberately prioritizes studies that utilize instrumental acoustic analysis or controlled perceptual experiments to assess the phonetic and phonological abilities of L2 learners. Although not aiming for an exhaustive review, the chapter explores the following major themes in the surveyed research literature: the role of L2 input quantity and quality, the influence of different phonological targets, the impact of explicit pronunciation instruction and corrective feedback, the extent and implications of individual differences, and the significance of L2 phonetic category formation and L1 restructuring in the classroom-based acquisition of L2 phonetics and phonology.