The Magen David (shield of David, or as it is more commonly known, the Star of David) is the symbol most commonly associated with Judaism today, but it is actually a relatively new Jewish symbol.  It is supposed to represent the shape of King David's shield (or perhaps the emblem on it), but there is really no support for that claim in any early rabbinic literature. In fact, the symbol is so rare in early Jewish literature and artwork that art dealers suspect forgery if they find the symbol in early works.

The use of the Magen David in a Jewish context as a possibly meaningful symbol may occur as early as the 11th century, in the decoration of the carpet page of the famous Tanakh manuscript, the Leningrad codex dated 1008.  Similarly, the symbol illuminates a medieval Tanakh manuscript dated 1307 belonging to Rabbi Yosef bar Yehuda ben Marvas from Toledo, Spain. 

It is in 16th century Prague that we begin seeing the Magen David as a Jewish symbol with some frequency.  A Siddur dated 1512 from Prague displays a large hexagram on the cover with the phrase, "…He will merit to bestow a bountiful gift on anyone who grasps the Shield of David."  It appeared on the pennants of Jewish guilds, including the Jewish Butchers Gild.  The Magen David is featured on a Parochet, a synagogue Torah ark curtain dated 1595,  It was in the 16th century that it made its appearance on the flag of the Jewish community of Prague, which now hangs in Prague's leading synagogue, known as the Altneuschul Shul.  Thereafter it appeared with some regularity in Jewsish ceremonial art objects for the synagoue and the home in Prague, including Hanukah lamps, and eventually even on decorative keys to synagogues, and more mundane objects, such as baby cradles.

Scholars such as Franz Rosenswieg have attributed deep theological significance to the symbol.  For example, some note that the top triangle strives upward, toward G-D, while the lower triangle strives downward, toward the real world.  Some note that the intertwining makes the triangles inseparable, like the Jewish people.  Some say that the three sides represent the three types of Jews: Kohanim, Levites and Israel. While these theories are theologically interesting, they have little basis in historical fact.

The symbol of intertwined equilateral triangles is a common one in the Middle East and North Africa, and is thought to bring good luck.  It appears occasionally in early Jewish artwork, but never as an exclusively Jewish symbol.  The nearest thing to an "official" Jewish symbol at the time was the menorah.

In the middle ages, Jews often were required to wear badges to identify themselves as Jews, much as they were in Nazi Germany, but these Jewish badges were not always the familiar Magen David.  For example, a fifteenth century painting by Nuno Goncalves features a rabbi wearing a six-pointed badge that looks more or less like an asterisk.

In the 17th century, it became a popular practice to put Magen Davids on the outside of synagogues, to identify them as Jewish houses of worship in much the same way that a cross identified a Christian house of worship.

The Magen David gained popularity as a symbol of Judaism when it was adopted as the emblem of the Zionist movement in 1897, but the symbol continued to be controversial for many years afterward. When the modern state of Israel was founded, there was much debate over whether this symbol should be used on the flag.

Today, the Magen David is a universally recognized symbol of Jewry.  It appears on the flag of the state of Israel, and the Israeli equivalent of the Red Cross is known as the Magen David Edom.