Abstract This research synthesis builds upon research related to the area of learning outcomes when using the source language in the ESL/EFL classroom. Evidence were achieved from articles from different journals that have been analyzed with the purpose to examine studies conducted in using the source language in the ESL/EFL classroom. The methods used consist of online database searches and search engines, as well as analyzing them accordingly to the research question. Conclusion declared that within second language learning contexts certain types of concepts e.g. translation and clarifying vocabulary are more frequent and research on language development reflects this bias. In this much debated phenomenon the use of the first language is discussed, whether it should be used in the ESL/EFL classroom or not. Furthermore when, how as well as to which extent the source language should be used. The results revealed that there are a major learning outcomes; such as students’ were able to comprehend the language patterns and functions and teachers need to learn more regarding the use of the L1 in the ESL/EFL classroom. Keywords: ESL/EFL classroom, L1 use, teachers’ code-switching