This paper is concerned with overviewing and evaluating the problem of information sharing in oligopoly, a new topic in contemporary economics. It is intended as a synthesis of the Economics of Imperfect Competition and the Economics of Uncertainty and Information. The problem at issue is how and to what extent an information transmission agreement among firms influences the welfare of producers, consumers and the whole society. It is seen that an answer to the problem depends on many factors. They are: the type of competitors (Cournot or Bertrand), the nature of risks (a common value or private values, demand or cost), the degree and direction of physical and stochastic interdependence among firms, and the number of participating firms. If any set of those factors is specified in a given oligopoly model, then the welfare and policy implications may systematically be derived through their decompsotion into own and cross variation effects, and into own and cross efficiency effects. This survey paper is divided into two parts. Part I first discusses duality between Cournot and Bertrand duopoly models in the absence of uncertainty, and then proceeds to focus on duopoly models facing a common risk. Part II turns to the case of private risks, and investigates the welfare impact of increasing the number of firms.