This small book has been written in order to make the teaching of S. John of the Cross on Interior Prayer more widely known. It also seemed fitting to offer it to the General of the Order of Discalced Carmelites. As is well known, this Order was reformed by S. Teresa with the help of S. John of the Cross. Among them the doctrine of the great Spanish mystic should be known to perfection, and they are competent to judge whether we have set it forth perfectly and faithfully.