The major purpose of this study is to analyze the distinction between Pareto's " ophelimity " and " utility " theories, a distinction which has been overlooked in the literature on Pareto. This paper will also serve to bring out the reasons why the distinction was of importance to Pareto himself. Essentially, the distinction between Pareto's ophelimity and utility theories has a methodological foundation, specifically, his views on scope. The paper is organized into three main parts. The first part presents Pareto's views on scope. The second part focuses attention on the equilibrium of systems defined in terms of scope. Finally, Pareto's ophelimity and utility theories are used as illustrations of Paretian welfare theory applied to two different systems.