This book is an endeavor to introduce the main reasons for shifting codes while speaking in an English-based classroom. The most prominent patterns of code-switching in specialized courses of Teaching English as a Foreign Language have been found and reasons for the implemented moments were discussed. Merits and demerits of conducting code-switching in post-graduate university levels have been brought up and suggestions have been provided for coursebook designers, test assessors, and code-switching users. In this inquiry which is an ethnographic exploratory work, instruments such as classroom observation, questionnaires, field notes, stimulated recall and interviews have been used in order to accumulate date.