This dissertation investigates the longitudinal development of Chinese language proficiency among students in a Chinese as a Foreign Language (CFL) Flagship Program at an American public university from 2014 to 2024. Using Bayesian multilevel regression models, this study assesses the trajectories of listening, speaking, and reading proficiency across various instructional contexts while accounting for critical factors such as motivation, heritage background, prior exposure, and the effects of the COVID-19 pandemic. Grounded in Complex Dynamic Systems Theory and Sociocultural Theory, this research contributes to the limited body of work on proficiency growth in less commonly taught languages by highlighting the complex nature of CFL acquisition. The analysis addresses three primary research questions: the average trajectories of proficiency development, differences in growth across language skills, and the influence of individual, environmental, and instructional factors on proficiency outcomes. Findings reveal that listening and speaking followed nonlinear growth patterns, while reading proficiency showed a more stable, linear progression. COVID-19 had a nuanced impact, slightly enhancing listening and reading growth, though its effect on speaking was mixed. Additionally, reading proficiency exhibited the fastest growth, ultimately converging with listening, whereas speaking proficiency maintained consistently higher starting and end points. Motivation emerged as a pivotal factor influencing growth across all skills, and heritage learners and students with prior exposure demonstrated early proficiency advantages, though these differences diminished over time. Study abroad or not itself didn't affect the growth rate of CFL learners but together with instructional context, it played an important role. Instructional context also impacted outcomes: blended learning (BL) enhanced speaking proficiency, online learning (OL) supported reading, and face-to-face (F2F) instruction benefited listening, particularly during study abroad. This study highlights the importance of understanding CFL acquisition as an interaction between individual traits, instructional context, and environmental factors. The findings have practical implications for CFL curriculum design, suggesting tailored approaches to address the diverse needs of learners. However, limitations such as the specific demographic and institutional context indicate a need for further research to validate these results across broader settings. This dissertation contributes to the methodology of L2 research, the growing understanding of effective instructional methods, and contextual factors that facilitate proficiency growth in critical language education. [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.]