The primary purpose of this investigation was to put forward critical research synthesis as a qualitative alternative to meta-analysis in second language acquisition using as a case example studies published from 2005 to 2018 applying the L2 Motivational Self System (L2MSS) to learners of languages other than English (LOTEs). Since L2MSS research is methodologically diverse and meta-analysis necessarily excludes literature that cannot be subjected to its parameters, a qualitative synthetic approach with the systematicity and transparency of meta-analysis is warranted. This study synthesizes 30 L2MSS studies on LOTEs in three world regions--Europe, Inner Circle English-speaking countries, and Asia--and examines substantive and methodological features of the literature, including (1) the diversity of languages and educational contexts, (2) common issues of interest and findings in each world region, and (3) the strengths and limitations of quantitative and qualitative L2MSS studies. Suggestions for improving the diversity and quality of research on motivation to learn LOTEs are offered.