Anyone who walks down a street in Israel or in Jewish communities in the Diaspora will notice that an odd looking rectangular ornament on each and every doorpost.  This ornament, called aMezuzah, is a commandment that most Jews of all religious levels and affiliations perform.

The word Mezuzah means "doorpost" in Hebrew.  This is appropriate because this is where the mitzvah, or commandment, is performed.   The mitzvah as stated in the Torah states that Jews are to "inscribe these words upon the doorposts of [your] houses and gates".  Themezuzahis therefore to be placed on the doorpost.  Jewish law states that the Mezuzah is to be placed on the top third of the doorpost on the right side, at the minimum immediately upon moving into a new home or within thirty days.  The general custom in theDiasporais to give thirty days with a preference for putting a mezuzah on the doorpost as soon as possible.  In Israel the custom is to put up a mezuzah immediately.  However, the general custom in Judaism is to place mezuzot on all doorposts in the house other than those leading to the bathroom because it is not respectful to place the words of the Torah by a bathroom.

In terms of the contents of the mezuzah, it is the first two paragraphs of theShema prayer, or more appropriately, declarative statement.  The Shema is a declaration of faith in G-d that Jews say twice a day.  These two paragraphs are written on parchment by a scribe who has special training to write mezuzahs.  The source for writing these two paragraphs is the Shema itself, when the Torah says what was stated above; that "You shall inscribe these words upon the doorposts of your houses and gates".  The mezuzah is generally checked twice every seven years although there is a tendency to have them checked more often because weather does cause the text and parchment of the Mezuzah to wear down.

In Judaism, a Mezuzah is considered to be spiritual and physical protection against harm.  The source for this is the Exodus, when the Jews placed blood on the doorposts so that the angel that killed the firstborn would not kill any Jews.  In addition, the Mezuzah's presence is supposed to remind those living in the house that G-d is present.   Jewish mysticism, better known as Kabbalah (This is not the Madonna-style spirituality that has become in vogue recently), states simply that when the mitzvah is carried out respectfully, the Mezuzah denies evil and destructive agents access to the house and those who put up a mezuzah are protected all the time.  In addition, when people treat a mezuzah with the proper respect, angels will accompany him wherever he goes and will protect him from danger.  However, this is not just a mystical concept.  The Code of Jewish Law, also known as the Shulchan Arukh, states that those who those who put Mezuzot on the doorposts of their houses and treat the mezuzah with respect will have their and their children's' lives lengthened.