This study outlines Ferguson’s (1959) classical diglossia and Fishman’s (1967) extended diglossia and refers to the modifications and extensions this concept has undergone since Ferguson’s (1959) original definition. The purpose is to show how Ferguson’s diglossia differs from the extensions formulated by other linguists and discuss the various critiques that the theory of diglossia has received in the years that followed. As stated by Ferguson (1959) himself, classical diglossia was intended to describe only the cases where genetically related varieties are used. Therefore, I argue that diglossia should maintain its original meaning so that a coherent theory of diglossia can be created by focusing on the implications and outcomes that such diglossic societies can have in relation to other sociolinguistic phenomena (such as identity, language attitudes, and language contact). Mixing diglossic and bilingual/multilingual cases because the language varieties involved are in functional distribution, possibly makes each phenomenon less valuable as they deserve special investigation and, a theory of their own.