We examined the naturalistic pronunciation development of two groups of L2 speakers over 10 years. Initially, 50 beginner ESL students participated in production tasks; despite attrition, the tasks were administered eight more times. Here we report listener judgements of accentedness, comprehensibility and fluency for the remaining six Mandarin and 12 Slavic language speakers at Year 10. Analyses of listener judgments of accentedness, comprehensibility, and fluency of utterances recorded at the 2-month, 1-year, 2-year, 7-year and 10-year points revealed that the Slavic language speakers improved in comprehensibility and fluency at each comparison point, while the Mandarin speakers' results were variable; there was improvement in comprehensibility from Year 7 to Year 10, but only after worsening at earlier points. The Slavic language group showed improvement in accentedness several times, whereas the Mandarin group showed no improvement in accentedness at any point. The data were examined for individual differences in learning trajectories. Interview responses and a survey of language use were compared to participants' trajectories. Some speakers showed steady improvement from Year 7 to Year 10, but the majority plateaued or regressed. We also elicited speakers' views of their progress. The results are interpreted through Complexity Theory and the Willingness to Communicate framework. Suggestions are made for research and teaching interventions.