The orthography of the modern Greek language was standardised in 1976 and simplified the diacritics in 1982. There are relatively few differences between the orthography of Ancient Greek and Modern Greek.

Some time prior to that, one early form of Greek, Mycenaean, was written in Linear B, although there was a lapse of several centuries (the Greek Dark Ages) between the time Mycenaean stopped being written and the time when the Greek alphabet came into use.

Early Greek writing in the Greek alphabet was phonemic, different in each dialect. Since the adoption of the Ionic variant for Attic in 403 BC, however, Greek orthography has been largely conservative and historical.

Given the phonetic development of Greek, especially in the Hellenistic period, certain modern vowel phonemes have multiple orthographic realizations:

This affects not only lexical items but also inflectional affixes, so correct orthography requires mastery of formal grammar, e.g. η καλή /i kaˈli/ 'the good one (fem. sing.)' vs. οι καλοί /i kaˈli/ 'the good ones (masc. pl.)'; καλώ /kaˈlo/ 'I call' vs. καλό /kaˈlo/ 'good (neut. sing.)'.

Similarly, the orthography preserves ancient doubled consonants, though these are now pronounced the same as single consonants, except in Cypriot Greek.

A digraph is a pair of letters used to write one sound or a combination of sounds that does not correspond to the written letters in sequence. The orthography of Greek includes several digraphs, including various pairs of vowel letters that used to be pronounced as diphthongs but have been shortened to monophthongs in pronunciation. Many of these are characteristic developments of modern Greek, but some were already present in Classical Greek. None of them is regarded as a letter of the alphabet.

During the Byzantine period, it became customary to write the silent iota in digraphs as an iota subscript.