The flag of Greece (Greek: Σημαία της Ελλάδος, popularly referred to as the Γαλανόλευκη or the Κυανόλευκη, the "blue-white") is based on nine 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 Christianity, the established religion of the majority of the people. According to popular tradition, the nine stripes represent the nine syllables of the phrase "Ελευθερία ή Θάνατος" ("Freedom or Death", " E-lef-the-ri-a i Tha-na-tos"), the five blue stripes for the syllables "Έλευθερία" and the four white stripes "ή Θάνατος". There is also a different theory, that the nine stripes symbolize the nine Muses, the goddesses of art and civilization (nine has traditionally been one of the numbers of reference for the Greeks).
The blazon of the flag is Azure, four bars Argent; the canton Azure with a Greek cross throughout Argent. 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.
The above patterns were officially adopted by the First National Assembly at Epidaurus in January 1822. Blue and white have many interpretations, symbolizing the colors of the famed Greek sky and sea (combined with the white clouds and waves), traditional colors of Greek clothes in the islands and the mainland, etc.

History of the Greek flag
White and blue have been symbolic Greek colors since antiquity with historic significance; their adoption for the new Greek state was a natural continuation from previous uses. In ancient Greece they were connected with goddess Athena and were used in Alexander the Great's army banners, while Greeks abroad were often recognized by their white clothes with blue details. They were even referred as colors connected with Greeks by Herodotus. During Byzantine times white and blue were the colors of navy and other flags, coats of arms of imperial dynasties, uniforms, Emperors' clothes, Patriarchs' thrones etc. The cross was a symbol of the empire, and was a common pattern in Byzantine flags since the 4th century.
The blue cross on a white field seems to have been the most "consistent" pattern. This design, including the four (blue) B's on the flag quarters has been a very important Byzantine symbol (the four B's represent the standard Byzantine motto, standing for Βασιλεύς Βασιλέων Βασιλεύων Βασιλευόντων, meaning "King of Kings Reigning over those who Rule", at times also interpreted as standing for Βασιλεύ Βασιλέων Βασιλέα Βοήθει, meaning "King of Kings, Save the King" - the B's often shaped in ways simultaneously representing other images). It appeared as early as (according to some sources) the 4th century, mainly as the Byzantine navy flag, apparently in "forked" shape, influencing other Byzantine emblems. It also appeared as one of the imperial flags during the last dynasty of the Empire, the Palaeologi (13th-15th centuries), featuring the blue cross and the four blue B's on a white field, as well as four small diagonal golden "beams". In that respect it differed from Palaeologid personal or dynasty flags that featured the traditional "imperial" colors, gold and/or red. Earlier, during the reign of Nicephorus II Phocas (963-969) and his successors, even those "imperial" colors had changed to blue and white. The official Army flag had become a white eagle on a blue field, while the imperial standard used the same colors, including the cross and/or blue and white stripes.

Greek flags during Antiquity and the Byzantine Empire
During the Ottoman rule several unofficial flags were used by Greeks, usually employing the double-headed eagle (see below), the cross, depictions of saints and various mottoes. The Greek Spachides cavalry employed by the Ottoman Sultan were allowed to use their own, clearly Christian flag, when within Epirus and the Peloponnese. It featured the classic blue cross on a white field with the picture of St. George slaying the dragon, and was used from 1431 until 1639, when this privilege was greatly limited by the Sultan. Similar flags were used by other local leaders. The closest to a Greek "national" flag during Ottoman rule was the Graikothomaniki pantiera, a flag Greek Orthodox merchants were allowed to fly on their ships, combining stripes with red (for the Ottoman Empire) and blue (for the Greeks) colors. After the Treaty of Küçük Kaynarca, Greek-owned merchant ships could also fly the Russian flag.
During the uprising of 1769 the historic blue cross on white field was used again by key military leaders who used it all the way to the revolution of 1821. It became the most popular Revolution, and it was argued that it should become the national flag. The "reverse" arrangement, white cross on a blue field, also appeared as Greek flag during the uprisings. This design had been used earlier as well, in Byzantine emblems, as a local symbol (a similar 16th or 17th century flag has been found near Chania), while Greek volunteers in Napoleon's army in Egypt in 1798 used a white cross on blue incorporated in the canton of the French flag.
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.

Ottoman period
In addition to the various cross flags, Greek intellectuals in Europe, as well as local leaders, chieftains and regional councils, designed and used flags with different colors and emblems during the early days of the Greek Revolution. Many of these flags featured saints, the phoenix (symbolizing the rebirth of the Greek nation), mottoes such as "Freedom or Death" (Ελευθερία ή Θάνατος) or the fasces-like emblems of the Philiki Etaireia.
Because the European monarchies, allied 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, took steps to portray revolutionary Greece as a "conventional", ordered nation-state. As such not only were the regional councils abolished in favor of a central administration, but it was decided to abolish all revolutionary flags and adopt a national flag. Why the particular arrangement (white cross on blue) was selected instead of the more popular blue cross on a white field, remains unknown.
On March 15, 1822, the Provisional Government, by Decree Nr. 540, laid down the exact pattern: white cross on blue (plain) for the land flag; nine alternate-coloured stripes with the white cross on a blue field in the canton for the naval flag; and blue cross on white in the canton of an otherwise blue flag, for the merchant navy. In 1828 the latter was discontinued, and the cross-and-stripes became the sole sea flag. This design became immediately very popular with Greeks and in practice was often used simultaneously with the national (plain cross) flag.
King Otto added the royal Coat of Arms (a shield in the Bavarian colors topped by a crown) to the center of the cross for military flags (both land and sea versions). After Otto's abdication in 1862, the royal coat of arms was removed, only to be replaced by a simple royal crown in 1863. A square version of the land flag with St. George in the center was adopted in April 9, 1864 as the Army's colours. Similar arrangements were made for the royal flags, which featured the coat of arms of the House of Schleswig-Holstein-Sonderburg-Glücksburg on a square version of the national flag. The exact shape and usage of the flags was determined by Royal Decree on September 27, 1867. By a new Royal Decree, on 31 May 1914, the flag with the crown was adopted for use as a state flag by ministries, embassies and civil services, while the merchant naval flag was allowed for use by private citizens.
On March 25, 1924, with the establishment of the Second Hellenic Republic, the crowns were removed from all flags. On February 20, 1930, the national flag's proportions were established at a 2:3 ratio, with the arms of the cross being "one fifth of the flag's width". The national ("land") flag was to be used by ministries, embassies, and in general by all civil and military services, while the naval flag was to be used by naval and merchant vessels, consulates and private citizens. With the restoration of the monarchy, on October 10, 1935, the crown was restored on the flags. The crown was again removed by the military dictatorship in 1967, and the naval flag was established as the sole national flag in 1969, using a dark shade of blue. On August 18, 1970, the flag ratio was changed to 7:12.
After the metapolitefsi, the land flag was restored for a while (Law 48/1975 and Presidential Decree 515/1975) until 1978, when the sea flag was re-adopted as the sole national flag, with the 2:3 ratio (Law 851/21-12-1978 "On the national Flag, War Flags and the Distinguishing Flag of the President of the Republic"). The land flag survives now mostly in military flags, as laid down by the Presidential Decree 348/17-4-1980. The Army's flag, as mentioned, features St. George, and the Air Force, similarly of square shape, the archangel Michael. The simple cross pattern is still used in the National Emblem of Greece, as the Navy's jack and in rank flags. School flags also follow the military pattern. The land flag continues to be flown in the Old Parliament in Athens, and can still be seen displayed by private citizens. In official use, the flagpole is topped by a white cross. The Greek Flag Day is on October 27.

Adoption of the flag and historical evolution
It may look surprising that one of the most recognizable (other than the cross) and beloved Greek symbols, the double-headed eagle, is not a part of the modern Greek flag or coat of arms (although it is officially used by the Greek Army, by the Church of Greece, and was incorporated in the Greek coat of arms between 1925 and 1926). One suggested explanation is that, upon independence, an effort was made for political - and international relations - reasons to limit expressions implying efforts to recreate the Byzantine empire. Yet another theory is that this symbol was only connected with a particular period of Greek history (Byzantine) and a particular form of rule (imperial). More recent research has justified this view, connecting this symbol only to personal and dynastic emblems of Byzantine Emperors.
Greek scholars have tried to make links with ancient symbols: the eagle was a common design representing power in ancient city-states, while there was an implication of a "dual-eagle" concept in the tale that Zeus left two eagles fly east and west from the ends of the world, eventually meeting in Delphi, thus proving it to be the center of the earth. However, there is virtually no doubt that its origin is a blend of Roman and Eastern influences. Indeed, the early Byzantine Empire inherited the Roman eagle (extended wings, head facing right) as an imperial symbol. During his reign, Emperor Isaac I Comnenus (1057–1059) modified it as double-headed, influenced by traditions about such a beast in his native Paphlagonia in Asia Minor (in turn reflecting possibly much older local myths). Many modifications followed in flag details, often combined with the cross. After the recapture of Constantinople by the Byzantine Greeks in 1261, two crowns were added (over each head) representing - according to the most prevalent theory - the newly recaptured capital and the intermediate "capital" of the empire of Nicaea. There has been some confusion about the exact use of this symbol by the Byzantines; it is quite certain that it was a "dynastic" and not a "state" symbol (a term not fully applicable at the time, anyway), and for this reason, the colors connected with it were clearly the colors of "imperial power", i.e., imperial purple and gold.
After the Ottoman conquest of Constantinople nonetheless, the double-headed eagle became a strong "national" symbol of reference for the Greeks (much less, though, than the cross), featured in several flag designs, especially during uprisings and revolts. Most characteristically, the Orthodox Church kept, and is to this date still using Byzantine flags with the eagle, usually black on yellow/gold background. But after the Ottoman conquest this symbol also found its way to a "new Constantinople" (or Third Rome), i.e. Moscow. Russia, deeply influenced by the Byzantine Empire, saw herself as its heir and adopted the double-headed eagle as its imperial symbol. It was also adopted by the Serbs, the Montenegrins, the Albanians and a number of Western rulers, most notably in Germany and Austria.