The revised Speech Learning Model (SLM-r) postulates that learners with more precisely defined categories in their native language (L1) exhibit greater proficiency in acquiring sounds in a second language (L2). Despite its recent emergence, empirical studies validating this hypothesis remain scarce. This study aims to investigate the predictive role of L1 category precision in L2 performance, particularly as a fundamental factor influencing L2 speech development. Furthermore, it aims to uncover a potential correlation between learners' perceptual and production abilities, mediated by L1 category precision, within the framework of L2 speech development. The findings from Studies 1 and 2 furnish compelling evidence supporting the impact of L1 category precision on the trajectory of L2 speech development, irrespective of linguistic factors. However, Study 3, which examines the relationship between perception and production influenced by L1 category precision, suggests that while production-based assessments of L1 category precision significantly predict L2 production, they may not strongly predict L2 perception. This indicates a tenuous connection between production and perception driven by individual differences in category formation. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]