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In this thesis, I focus on the use of code-switching (CS) - the use of more than one language in a stretch of discourse - in the English as a foreign language (EFL) classroom. Researchers have failed to reach a consensus on the effect of teachers' CS in the EFL classroom; some studies show that CS between the native and target language facilitates learning (e.g., Blom & Gumperz, 1972; Stern, Allen & Harley, 1992; Cook, 2001, 2008; inter alia), while others suggest the opposite (e.g., Chaudron, 1988; Ellis, 1984; Wong-Fillmore, 1985; Halliwell & Jones, 1991; Chambers, 1991; and Macdonald, 1993). These differences may arise from the various purposes for which a teacher uses CS, whether it be for classroom management, linguistic explanation, or, more solely for social purposes. In this study, I allow for the possibility of differential effects of various types of CS by distinguishing two broad categories and applying them in the classroom. The first category is Methodological CS, where CS is employed to explain linguistic phenomena only. The second category is Mixed CS, where the move between two languages takes place across linguistic, classroom management, and social purposes. I test the effect of these different categories of CS on students' outcomes of learning English skills/components (vocabulary, grammar, reading, writing, listening) across four different age levels (elementary, intermediate, secondary, and university). Within each level, I taught three different groups for eight hours each, using a different category of CS for each class. Methodological CS was used with one group, while Mixed CS was used with another group, and one control group experienced no CS. A linear regression model of the differences between the pre-test and post-test exam scores revealed that students' performance improved more in the two CS groups than in the control group, regardless of the CS category. However, the benefit of CS was generally higher in the Mixed CS group than in the Methodological CS group. The interaction between the age and CS categories showed that the largest improvements were in the groups where Mixed CS was used, especially at the intermediate level, whereas the relationship between English skills and CS categories revealed the largest improvement in vocabulary and reading where Mixed CS was used. Therefore, Mixed CS generally helped students to improve their performance in English. To conclude, the findings suggest that a rich CS environment, using Mixed CS in particular, facilitates learning in the EFL classroom.