Literally, iconoclasm is the destruction of religious icons and other sacred images or monuments, usually for religious or political motives. In Christian circles, iconoclasm has generally been motivated by a literal interpretation of the second of the ten commandments, which forbids the making and worshipping of "graven images".
People who engage in such practices are called iconoclasts, a term that has come to be applied to any person who breaks or disdains established dogmas or conventions. Conversely, people who revere or venerate religious images are called iconodules.
Iconoclasms can be carried out by people of a different religion, but are often the result of sectarian disputes between factions of the same religion.
The first iconoclastic period: 730-787
Emperor Leo III the Isaurian (reigned 717–741) banned the use of icons of Jesus, Mary, and the Saints and commanded the destruction of these images in 730. The Iconoclastic Controversy was fueled by the refusal of many Christians resident outside the Byzantine Empire, including many Christians living in the Islamic Caliphate, to accept the emperor's theological arguments. St. John of Damascus was one of the most prominent of these. Ironically, Christians living under Muslim rule at this time had more freedom to write in defense of icons than did those living in the Byzantine Empire. Leo was able to promulgate his policy because of his personal popularity and military success — he was credited with saving Constantinople from an Arab siege in 717–718 and then sustaining the Empire through annual warfare.
Leo III's son, Constantine V (reigned 741–775) was challenged at once by a general who used Iconophilic ("Icon-favoring") propaganda, but his military success against this threat cemented his own position.
The first Iconoclastic period came to an end when Leo IV (Constantine V) died and his widow, Empress Irene came into power. An iconophile, she initiated the Second Council of Nicaea in 787, where the veneration of icons was affirmed, although the worship of icons was expressly forbidden. Among the reasons were the doctrine of the Incarnation: because God the Son (Jesus Christ) took on flesh, having a physical appearance, it is now possible to use physical matter to depict God the Son, and to depict the saints. Icon veneration lasted through the reign of Empress Irene's successor, Nicephorus I (reigned 802-811), and the two brief reigns after his. 
The second iconoclastic period: 813-843
Emperor Leo V (reigned 813–820) instituted a second period of Iconoclasm in 813, which seems to have been less rigorously enforced, since there were fewer martyrdoms and public destructions of icons. Leo was succeeded by Michael II, who was succeeded by his son, Theophilus. Theophilus died leaving his wife Theodora regent for his minor heir, Michael III. Like Irene 50 years before her, Theodora mobilized the iconodules and proclaimed the restoration of icons in 843. Since that time the first Sunday of Lent is celebrated in the churches of the Orthodox tradition as the feast of the "Triumph of Orthodoxy". 
Because of the prohibition against figural decoration in mosques — not, as is often said, a total ban on the use of images — some Muslim groups have on occasion committed acts of iconoclasm against the devotional images of other religions. A recent example of this is the 2001 destruction of frescoes and the monumental statues of the Buddha at Bamiyan by the radical Muslim sect and nationalist group, the Taliban.
Historically, despite a religious prohibition on destroying or converting houses of worship, conquering Muslim armies would on occasion replace local temples or houses of worship with mosques. An example is the Hagia Sophia, Church of the Holy Wisdom, in Istanbul, formerly Constantinople which was converted into a mosque 1453, when its mosaics were covered with plaster. The Dome of the Rock in Jerusalem is said to have been built on top of the remains of the Jewish Temple in Jerusalem. Some ultra-religious Jewish messianic groups believe that only by similarly demolishing the Dome of the Rock and rebuilding the Jewish Temple, can the messiah come to earth. This has led to frequent tension between Jews and Muslims over the site.
Similar acts of iconoclasm occurred in parts of north Africa.
In India, a number of former Buddhist monasteries and Hindu temples were conquered and rebuilt as mosques. In recent years, right-wing Hindu nationalists have torn down some of these mosques, such as the famous Babri Masjid, and attempted to replace them with Hindu Temples. 
Some of the Protestant reformers encouraged their followers to destroy Catholic art works by insisting that they were idols. Huldrych Zwingli and John Calvin promoted this approach to the adaptation of earlier buildings for Protestant worship. In 1562, some Calvinists destroyed the tomb of St. Irenaeus and the relics inside, which had been under the altar of a church since his martyrdom in 202, though iconoclastic riots took place in Zürich (in 1523), Copenhagen (1530), Münster (1534), Geneva (1535), Augsburg (1537) and Scotland (1559).
The Seventeen Provinces (now the Netherlands and Belgium) were hit by a large wave of Protestant iconoclasm in the summer of 1566. This is called the Beeldenstorm and included such acts as the destruction of the statuary of the Monastery of Saint Lawrence in Steenvoorde after a Hagenpreek, or field sermon, by Sebastiaan Matte; and the sacking of the Monastery of Saint Anthony after a sermon by Jacob de Buysere. The Beeldenstorm marked the start of the revolution against the Spanish forces and the Catholic church. See Flanders for more on its history.
In England, Bishop Joseph Hall of Norwich described the events of 1643 when troops and citizens, encouraged by a Parliamentary ordinance against superstition and idolatry, behaved thus:
'Lord what work was here! What clattering of glasses! What beating down of walls! What tearing up of monuments! What pulling down of seats! What wresting out of irons and brass from the windows! What defacing of arms! What demolishing of curious stonework! what tooting and piping upon organ pipes! And what a hideous triumph in the market-place before all the country, when all the mangled organ pipes, vestments, both copes and surplices, together with the leaden cross which had newly been sawn down from the Green-yard pulpit and the service-books and singing books that could be carried to the fire in the public market-place were heaped together'.