Neural plasticity in speech acquisition and learning is concerned with the timeline trajectory and the mechanisms of experience-driven changes in the neural circuits that support or disrupt linguistic function. In this selective review, we discuss the role of phonetic learning in language acquisition, the "critical period" of learning, the agents of neural plasticity, and the distinctiveness of linguistic systems in the brain. In particular, we argue for the necessity to look at brain-behavior connections using modern brain imaging techniques, seek explanations based on measures of neural sensitivity, neural efficiency, neural specificity and neural connectivity at the cortical level, and point out some key factors that may facilitate or limit second language learning. We conclude by highlighting the theoretical and practical issues for future studies and suggest ways to optimize language learning and treatment. [This work was supported in part by Department of Speech-Language-Hearing Sciences, University of Minnesota, Research Center for Advanced Technologies, Tokyo Denki University and Office of the Dean of the Graduate School, University of Minnesota.]