With recent changes in case law requiring district courts to make deeper inquiries at the class certification stage, a systematic treatment of the economics of class certification takes on great importance. This article offers an economic interpretation of the legal concept of common impact as it is used in antitrust class certification. It interprets the predominance requirement central to Rule 23(b)(3) to mean that all economically significant factors are common factors. These economic factors may or may not be related to the conduct alleged by plaintiffs; thus, common impact should be investigated by examining the defendant conduct that plaintiffs allege violated antitrust law (“conduct factors”) as well as to determinants of price unrelated to the alleged conduct (“non-conduct factors”). With this economic interpretation established, this article proceeds to examine empirical testing of common impact and describes three types of tests that are likely to be applicable in a wide variety of antitrust class certification matters. Each type of test focuses on differences in prices paid by putative class members.