In recent years scholars have begun to acknowledge that the occult sciences were not marginal enterprises but an integral part of the worldview of many of our ancestors. Astrology was one of the many intellectual tools--along with what we consider to be the superior tools of social and political analysis--that Renaissance thinkers used to attack practical and intellectual problems. It was a coherent body of practices, strongly supported by social institutions. And alchemy was not viewed primarily as a spiritual pursuit, an idea popularized by nineteenth-century occultists, but as a part of natural philosophy. It was often compared to medicine. Many Renaissance writers suggested links between astrology and alchemy that went beyond the use of astrological charts to determine the best time to attempt alchemical operations. This book shows the many ways in which astrology (a form of divination) and alchemy (an artisanal pursuit concerned with the technologies of minerals and metals) diverge as well as intersect. Overall, it shows how an appreciation of the role of the occult opens up new ways of understanding the past. Topics include the career of Renaissance astrologer Girolamo Cardano and his work on medical astrology, the astrological thinking of Johannes Kepler and Galileo Galilei, the history of the Rosicrucians and the influence of John Dee, the work of medical alchemist Simon Forman, and an extended critique of the existing historiography of alchemy.