The purpose of this chapter is to examine the astronomical knowledge, and more specifically the astronomical tools, that ancient astrologers in Mesopotamia and the Greco-Roman world possessed and used. Much of its content will be well known to specialists in ancient astronomy and astrology, but this is the first broad treatment of the topic. The roughly 1200-year evolution of astrological practice surveyed in this chapter is characterized by several shifts. First, interpretation of direct observations of the heavens was progressively supplanted by reliance on predicted astronomical data. Second, prediction based on the principle that astronomical phenomena observed in the past would approximately repeat after certain time intervals (called recurrence periods) gave way to mathematical models that had a more remote derivation from observations. Finally, astrologers became increasingly removed from the production of the astronomical information they used and increasingly dependent on published almanacs comprising precomputed data. This chapter is thus a contribution to understanding the expertise of an ancient astrologer as well as its limits.