Modern Greek (endonym: Νέα Ελληνικά, Néa Elliniká [ˈne.a eliniˈka] or Κοινή Νεοελληνική Γλώσσα, Kiní Neoellinikí Glóssa), generally referred to by speakers simply as Greek (Ελληνικά, Elliniká), refers collectively to the dialects of the Greek language spoken in the modern era, including the official standardized form of the language sometimes referred to as Standard Modern Greek. The end of the Medieval Greek period and the beginning of Modern Greek is often symbolically assigned to the fall of the Byzantine Empire in 1453, even though that date marks no clear linguistic boundary and many characteristic features of the modern language arose centuries earlier, having begun around the fourth century AD.

During most of the Modern Greek period, the language existed in a situation of diglossia, with regional spoken dialects existing side by side with learned, more archaic written forms, as with the vernacular and learned varieties (Dimotiki and Katharevousa) that co-existed in Greece throughout much of the 19th and 20th centuries.

Varieties of Modern Greek include Demotic, Katharevousa, Pontic, Cappadocian, Mariupolitan, Southern Italian, Yevanic, Tsakonian and Greco-Australian.

Strictly speaking, Demotic or Dimotiki (Δημοτική), refers to all popular varieties of Modern Greek that followed a common evolutionary path from Koine and have retained a high degree of mutual intelligibility to the present. As shown in Ptochoprodromic and Acritic poems, Demotic Greek was the vernacular already before the 11th century and called the "Roman" language of the Byzantine Greeks, notably in peninsular Greece, the Greek islands, coastal Asia Minor, Constantinople, and Cyprus.

Today, a standardized variety of Demotic Greek is the official language of Greece and Cyprus, and is referred to as "Standard Modern Greek", or less strictly simply as "Greek", "Modern Greek", or "Demotic".

Demotic Greek comprises various regional varieties with minor linguistic differences, mainly in phonology and vocabulary. Due to the high degree of mutual intelligibility of these varieties, Greek linguists refer to them as "idioms" of a wider "Demotic dialect", known as "Koine Modern Greek" (Koiní Neoellinikí - 'common Neo-Hellenic'). Most English-speaking linguists however refer to them as "dialects", emphasizing degrees of variation only when necessary. Demotic Greek varieties are divided into two main groups, Northern and Southern.

The main distinguishing feature common to Northern variants is a set of standard phonological shifts in unaccented vowel phonemes: [o] becomes [u], [e] becomes [i], and [i] and [u] are dropped. The dropped vowels' existence is implicit, and may affect surrounding phonemes: for example, a dropped [i] palatalizes preceding consonants, just like an [i] that is pronounced. Southern variants do not exhibit these phonological shifts.

Examples of Northern dialects are Rumelian (Constantinople), Epirote, Macedonian, Thessalian, Thracian, Northern Euboean, Sporades, Samos, Smyrna, and Sarakatsanika. The Southern category is divided into groups that include:^{[citation needed]}