A considerable period of time has passed since the manufacture of altar wine and the Church legislation concerning it was thoroughly discussed by Fr. Arthur McNeil, S.J., (1938)[i] and reviewed by Fr. Gerald Clark, O.P., (1943).[ii] Since that time there have been a number of developments in wine technology and at least one new ecclesiastical pronouncement.[iii] The Purpose of this article is to review the moral principles and canonical legislation concerning wine used for the Holy Sacrifice of the Mass, and to apply them to the various processes currently in use in this country in the production of wine, stressing particularly the ways in which wine may become illicit or invalid for use as altar wine. At the end, suggestions will be offered for judging a particular wine as valid and licit altar wine.