This book analyzes informal conversations of bilingual educated speakers of different urban varieties of Arabic, to investigate linguistic principles governing bilingual and diglossic code-switching, in terms of types, grammatical constraints, and function. ESA and CAIRENE ARABIC are investigated as to whether either is currently used as a LINGUA FRANCA common language used by speakers not sharing mutually intelligible languages or varieties) among Arabic speakers whose native varieties are different and may not be mutually intelligible. Also, differences in language and variety choice and code-switching strategies between genders are investigated. Linguistic perceptions and attitudes are also considered.