History
St. Catherine Laboure, of the Daughter of Charity, received visions of the Blessed Mother which were to result in the introduction of the Miraculous Medal. During the vision that took place on July 18, 1830, in the chapel of the motherhouse in Paris, France, the Blessed Mother sat on the director's chair near the altar. After confiding certain matters to St. Catherine, she said, "Come to the foot of the altar; there graces will be shed upon all, great and little, who ask for them. Graces will be especially shed upon those who ask for them."
During the vision that occurred on November 27, 1830, the Blessed Mother stood with her hands outstretched, brilliant rays of light coming from rings on her fingers. Some rings gave off no rays, representing graces for which souls forget to ask. Our Lady's feet were crushing the head of the serpent. An oval framed the Blessed Mother, showing these words in letters of gold: "O Mary conceived without sin, pray for us who have recourse to thee."
Promise
The a voice was heard to say: "Have a medal struck after this model. Those who wear it will receive great graces. They should wear it around the neck. Abundant graces will be given to those who wear it with confidence." The back of the medal was then revealed: a large M surmounted by a bar and a cross. Beneath the M were the hearts of Jesus and Mary, the one crowned with thorns, the other pierced with a sword. The whole was encircled with twelve stars.
The medal had originally been called the "medal of the Immaculate Conception," but so many remarkable graces and favors were soon granted through it that it quickly became known as the Miraculous Medal.
Source: Prayers and Heavenly Promises, Compiled from Approved Sources by John Carroll Cruz