This study provides the first longitudinally designed, classroom-based empirical test of self-determination theory's motivation mediation model. Measures of perceived autonomy support, motivation (autonomy need satisfaction), engagement, and achievement were collected from 500 (257 females, 243 males) 8th-grade students in Korea in a 3-wave longitudinal research design. Multilevel structural equation modeling tested the model in which early-semester perceived autonomy support increased mid-semester autonomy need satisfaction, which, in turn, increased end-of-the-semester engagement, which then predicted course achievement. We further tested for possible reciprocal pathways and for the stability of all effects throughout the model. Results revealed a complex, dynamic model that unfolds within naturally occurring classroom processes, one that validated the hypothesized model but also extended and qualified it in important ways. All hypothesized effects were supported, but they were not stable over the course of the semester, largely because of the emergence of several reciprocal effects. Overall, this longitudinal test revealed a more dynamic model than suggested by previous cross-sectional investigations. (Contains 3 tables, 2 figures and 2 footnotes.)