In this article, we set up an economic framework for analyzing the competitive effects of partial ownership interests. We have three main goals. First, we conceptually derive and explain the competitive effects of partial ownership, explaining its key elements and drawing analogies to the key ideas behind the analysis of horizontal mergers. Second, we present a general framework for evaluating the competitive effects of partial ownership that is analogous to, but at the same time recognizes key differences in the standard analysis for evaluating horizontal mergers. Third, we examine several methods of quantifying these competitive effects