Partnerships are growing rapidly in business, international development, and public policy generally. This phenomenon brings daunting but exciting challenges for evaluators. The first problem is defining just what partnership might mean. 1 As Robert Axelrod pointed out earlier in this book, there may be analogues to the Prisoner’s Dilemma, where proceeding without partnership leads both participants to worse outcomes than they might have achieved had they properly taken into account their strategic interaction. Partnerships may be analyzed in terms of gains from trade, where one party is good at wheat, the other at iron, or complementarities, where one is good at marketing, and the other at production, and we decide to join forces. In all these situations, one can speak of potential gains to be had from recognizing strategic interdependence, and therefore not thinking like myopic individuals.