In thirteenth-century Europe, astrology was a much-studied and often-condemned subject. Its place as one of the sciences was contingent upon the particular milieu in which it was practiced. For Roger Bacon, a Franciscan friar in Paris, it was not only an important subject but vital to the preservation of Christianity. However, astrology was often merged with black magic and other heretical practices, making a defense of it necessary. In his Opus maius, Bacon attempts to explain that astrology is not only safe to practice but is important because, without knowing the influences which come from the stars, a man is condemned to be controlled by them. This assertion illustrates Bacon’s views on man’s free will and how much power the stars exert on the sublunar realm. The issues of determinism and man’s free will are present in many other defenses of astrology, both contemporary with and prior to Bacon’s Opus maius. Bacon’s solution to preserving man’s free will while still ascribing power to the stars is one that bears interesting similarities to that of Abu Ma‘shar in Book I of his Kitab al-Madkhal al-kabir. Both men attempt to avoid the strict determinism that would have the stars controlling all aspects of a man’s life, removing his free will, while maintaining the tradition of particular stellar influence which had been a part of astrology at least since Ptolemy’s Tetrabiblos in the second century A.D. However, each solution, while achieving much the same goal, follows a different path which especially reveals the religious overtones of Bacon’s defense and his desire to give power to Christianity through the use and understanding of astrology.