The Arabic language is a difficult language to study due to the existence of two varieties within the same language; that is, its diglossic nature. Consequently, vocabulary acquisition in Arabic seems to suffer because both varieties known as standard Arabic and colloquial Arabic are acquired in different ways and are linguistically distant. Hence, the distance between the two varieties of Arabic affects vocabulary acquisition of the language. This book is based on a research study which explains the relationship between diglossia and vocabulary acquisition. This study includes two objectives. The first objective is to investigate the effect of diglossia on vocabulary acquisition of bilingual Lebanese students in Arabic. The second objective is to determine which vocabulary (Arabic vs. English) students have a preference for. The results showed a significant negative effect between diglossia and vocabulary acquisition. The students showed more confidence in using the English language over Arabic. This book would be useful to professionals in the education field, researchers, and anyone that is interested in the Arabic language.