The Greek alphabet is descended from the Phoenician alphabet, and is not related to Linear B or the Cypriot syllabary, earlier writing systems for Greek. It has given rise to many other alphabets used in Europe and the Middle East, including the Latin alphabet. In addition to being used for writing Modern Greek, its letters are today used as symbols in mathematics and science, as particle names in physics, as components of star names, in the names of fraternities and sororities, in the naming of supernumerary tropical cyclones, and for other purposes.

The Greek alphabet emerged in the mid-eighth century BC, centuries after the fall of the Mycenaean civilization and consequent abandonment of its Linear B script, an early Greek writing system. Linear B is descended from Linear A, which was developed by the Minoans, whose language was probably unrelated to the Greek language; consequently the Minoan syllabary did not provide an ideal medium for the transliteration of the sounds of the Greek language.

The Greek alphabet we recognize today arose after the Greek Dark Ages, i.e. the period between the downfall of Mycenae (ca. 1200 BC) and the rise of Ancient Greece, which begins with the appearance of the epics of Homer, around 800 BC, and the institution of the Ancient Olympic Games in 776 BC Its most notable change, as an adaptation of the Phoenician alphabet, is the introduction of vowel letters, without which Greek would be illegible.

Vowel signs were originally not used in Semitic alphabets. In the earlier West Semitic family of scripts (Phoenician, Hebrew, Moabite etc.), a letter always stood for a consonant in association with an unspecified vowel or no vowel. This did not reduce legibility because words in Semitic languages are based on triliteral roots that make meaning clear with only the consonants present, and vowels are clear from context. By contrast, Greek is an Indo-European language, and thus differences in vowels make for vast differences in meanings. Thus, the Greek alphabet divided the letters into two categories, consonants ("things that sound along") and vowels, where the consonant letters always had to be accompanied by vowels to create a pronounceable unit. Although the old Ugaritic alphabet did develop matres lectionis, i.e., use of consonant letters to denote vowels, they were never employed systematically.

Proto-Sinaitic alphabet 19 c. BCE

The first vowel letters were Α (alpha), Ε (epsilon), Ι (iota), Ο (omicron), and Υ (upsilon), modifications of Semitic glottal, pharyngeal, or glide consonants that were mostly superfluous in Greek: /ʔ/ ('aleph), /h/ (he), /j/ (yodh), /ʕ/ (ʿayin), and /w/ (waw), respectively. In eastern Greek, which lacked aspiration entirely, the letter Η (eta), from the Semitic glottal consonant /ħ/ (heth) was also used for the long vowel /ɛː/, and eventually the letter Ω (omega) was introduced for a long /ɔː/. The reason for the introduction of letters catering for long open e and o lies in the verbal morphology of the language. Classical Greek had a distinction between indicative and subjunctive made by alternating /e/ with /ɛː/ and /o/ with /ɔː/, which was made explicit by this innovation. The other vowels did not need a graphic length distinction.

Greek also introduced three new consonant letters, Φ (phi), Χ (chi) and Ψ (psi), appended to the end of the alphabet as they were developed. These consonants made up for the lack of comparable aspirates in Phoenician. In western Greek, Χ was used for /ks/ and Ψ for /kʰ/ — hence the value of the Latin letter X, derived from the western Greek alphabet. The origin of these letters is disputed.

The letter Ϻ (san) was used as a variant of Σ (sigma). By classical times sigma had won, and san disappeared from the alphabet. The letters Ϝ (wau, later called digamma) and Ϙ (qoppa) also fell into disuse. The former was only needed for the western dialects, and the latter was never truly needed at all. But these letters lived on in the Ionic numeral system, which consisted of writing a series of letters with precise numerical values. Ϡ (sampi), apparently a rare local glyph form from Ionia, was introduced at latter times to stand for 900. Thousands were written using a mark at the upper left ('A for 1000, etc.).

Because Greek minuscules arose at a much later date, no historic minuscule actually exists for san. Minuscule forms for the other letters were only used as numbers. For the number 6, modern Greeks use an old ligature called stigma (Ϛ, ϛ) instead of digamma, or ΣΤ/στ if this is not available. For 90 the modern Z-shaped qoppa forms were used: Ϟ, ϟ. (Note that some web browser/font combinations will show the other qoppa here.)

Originally there were several variants of the Greek alphabet, most importantly western (Chalcidian) and eastern (Ionic) Greek. The former gave rise to the Old Italic alphabet and thence to the Latin alphabet, while the latter is the basis of the present Greek alphabet. Athens originally used the Attic script for official documents such as laws and the works of Homer: this contained only the letters from alpha to upsilon, and used the letter eta for the sound "h" instead of the long "e". In 403 BC, Athens adopted the Ionic script as its standard, and soon the other versions disappeared.