Background: Although there are numerous success records of Content and Language Integrated Learning (CLIL) implementation in educational settings and its principles have been found effective, the impact of the whole educational methodology may be overrated. Method: A comprehensive search conducted to retrieve articles published between August 2005 and July 2016 using the scholarly databases. Results: We identified 102 potentially relevant articles of which only 6 met the inclusion criteria. Positive effect sizes in 5 studies indicated that the experimental group outperformed the control group. These effect sizes were analyzed separately to provide an interpretative context for the main results. High heterogeneity was observed (Q = 5, P < .001). The Chi-squared significance test shows that the distribution of effect sizes has heterogeneity. Likewise, I-squared statistic quantifies the heterogeneity on the data. Conclusion: Despite the positive feedbacks reported by researchers on the efficiency of such methodology applied in primary and secondary schools, it is occasionally admitted that the results of the study are in doubt as the participants of CLIL and non-CLIL groups do not have equal exposure to the foreign language. To compare both groups in the same conditions, it is necessary for both to receive the same number of instruction hours in L2. Article visualizations
