The present dissertation is a synthesis and meta-analysis of the 15-year-old research domain of the L2 Motivational Self System (L2MSS) model initiated by Dornyei (2005). The study is based on data from 100 studies involving 144 unique samples and 51,905 participants, and it addresses the inter-relationships of L2MSS elements (i.e., the Ideal L2 Self, the Ought-to L2 Self, the L2 Learning Experience) as well as their relationships with the two outcome variables of intended effort and language achievement. Based on the overall uncorrected and corrected true mean correlation estimates (r & [rho]), the results demonstrate that there is a strong positive association between intended effort and both the L2 Learning Experience (r = 0.62; [rho] = 0.79) and the Ideal L2 Self (r = 0.61; [rho] = 0.77), whereas its association with the Ought-to L2 self is of a small to medium (r = 0.37; [rho] = 0.49) strength. Moreover, L2 achievement is positively related to the L2 Learning Experience (r = 0.27; [rho] = 0.31) and the Ideal L2 Self (r = 0.24; [rho] = 0.26), but both with a small effect, whereas there is no true association between L2 achievement and the Ought-to L2 Self (r = 0.01; [rho] = 0.02). Lastly, the L2MSS elements are all positively correlated with one another (rs = 0.51, 0.34, & 0.31). Among the ten potential moderators examined empirically in the meta-analysis, four were found to be significant in systematically affecting one or more focused relationships: national context, target L2, gender, and publication status. The other six variables (language learners' age, major, language context, rural vs. urban differences, L2 vs. L3 differences, and the Likert scales used) had no clear moderating effects. The synthetic findings point to the worldwide surging popularity and dominance (since 2015) of the L2MSS model in the field of L2 motivation. Despite an increasing interest in studying L2 Mandarin motivation shown in some studies (11%), the majority of the L2MSS studies overwhelmingly investigated college-level EFL learners from Asian countries, and this and other contextual imbalances limit the generalizability of the model. Finally, it was revealed that the L2 Learning Experience and language achievement are comparatively unstudied variables. Other areas in need of attention in the future in primary studies in this domain are the lack of investigation of certain moderating variables of L2 motivation (e.g., L2 vs. L3 differences), and insufficiently thorough reporting conventions. [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.]