The national flag of Greece, popularly referred to as the Blue-and-White (Γαλανόλευκη, Galanólefki) or the Cyan-and-White (Κυανόλευκη, Kyanólefki), is officially recognised by Greece as one of its national symbols and has 5 equal horizontal stripes of blue alternating with white. There is a blue canton in the upper hoist-side corner bearing a white cross; the cross symbolises Eastern Orthodox Christianity. The blazon of the flag is azure, four bars argent; on a canton of the field a Greek cross throughout of the second. The shade of blue used in the flag has varied throughout its history, from light blue to dark blue, the latter being increasingly used since the late 1960s. It was officially adopted by the First National Assembly at Epidaurus on 13 January 1822.

While the nine stripes do not have any official meaning, the most popular interpretation says that they represent the syllables of the phrase Ελευθερία ή Θάνατος (Freedom or Death): the five blue stripes for the syllables in Ελευθερία, the four white for those of ή Θάνατος. White and blue symbolize the colours of the Greek sky and sea.

It has been suggested by historians that the current flag derived from an older design, the virtually identical flag of the powerful Cretan Kallergis family. This flag was based on their coat of arms, whose pattern is supposed to be derived from the standards of their claimed ancestor, Byzantine Emperor Nicephorus II Phocas (963–969 AD). This pattern (according to not easily verifiable descriptions) included nine stripes of alternating blue and white, as well as a cross, assumed to be placed on the upper left. Although the use of alternating blue and white – or silver – stripes on (several centuries-old) Kallergis' coats of arms is well documented, no depiction of the above described pattern (with the nine stripes and the cross) survives.

The Byzantines, like the Romans before them, used a variety of flags and banners, primarily to denote different military units. These were generally square or rectangular, with a number of streamers attached. Most prominent among the early Byzantine flags was the labarum. In the surviving pictorial sources of the middle and later Empire, primarily the illustrated Skylitzes Chronicle, the predominating colours are red and blue in horizontal stripes, with a cross often placed in the centre of the flag. Other common symbols, prominently featuring on seals, were depictions of Christ, the Virgin Mary and saints, but these represent personal rather than family or state symbols. Western European-style heraldry was largely unknown until the last centuries of the Empire.

There is no mention of any "state" flag until the mid-14th century, when a Spanish atlas, the Conosçimiento de todos los reynos depicts the flag of "the Empire of Constantinople" combining the red-on-white Cross of St George with the "tetragrammatic cross" of the ruling house of the Palaiologoi, featuring the four betas or pyrekvola ("fire-steels") on the flag quarters representing the imperial motto Βασιλεύς Βασιλέων Βασιλεύων Βασιλευόντων ("King of Kings Reigning over those who Rule"). The tetragrammatic cross flag, as it appears in quarters II and III in this design, is well documented. In the same Spanish atlas this "plain" tetragrammatic cross flag is presented as (among other places in the Empire) "the Flag of Salonika" and "the real Greece and Empire of the Greeks (la vera Grecia e el imperio de los griegos)". The (quartered) arrangement that includes the Cross of St. George is documented only in the Spanish atlas, and most probably combines the arms of Genoa (which had occupied Galata) with those of the Byzantine Empire, and was most probably flown only in Constantinople. Pseudo-Kodinos records the use of the "tetragrammatic cross" on the banner (phlamoulon) borne by imperial naval vessels, while the megas doux displayed an image of the emperor on horseback.

During the Ottoman rule several unofficial flags were used by Greeks, usually employing the Byzantine double-headed eagle (see below), the cross, depictions of saints and various mottoes.

A military leader, Yiannis Stathas, used a flag with white cross on blue on his ship since 1800. The first flag featuring the design eventually adopted was created and hoisted in the Evangelistria monastery in Skiathos in 1807. Several prominent military leaders (including Theodoros Kolokotronis and Andreas Miaoulis) had gathered there for consultation concerning an uprising, and they were sworn to this flag by the local bishop.

European monarchies, aligned in the so-called "Concert of Europe", were suspicious towards national or social revolutionary movements such as the Etaireia. The First Greek National Assembly, convening in January 1822, thus took steps to disassociate itself from the Etaireia's legacy and portray nascent Greece as a "conventional", ordered nation-state. As such, not only were the regional councils abolished in favour of a central administration, but it was decided to abolish all revolutionary flags and adopt a universal national flag. The reasons why the particular arrangement (white cross on blue) was selected, instead of the more popular blue cross on a white field, remain unknown.