The Apology of the Augsburg Confession was formulated by Philipp Melanchthon as the response to the Roman Confutation against the Augsburg Confession. Melanchthon was writing a defense of the original Confession, and a refutation of this Confutation that Emperor Charles V had commissioned.
The second edition, published in September 1531 in Latin is the edition that is most widely recoginzed as the best and includes changes to the original that were suggested by Martin Luther. The Book of Concord includes it as a Lutheran confessional document. It is seven times longer than the Augsburg Confession itself.