In this paper I seek to answer two interrelated questions about pleasures and pains: (i) The question of unity: Do all pleasures share a single quality that accounts for why these, and only these, are pleasures, and do all pains share a single quality that accounts for why these, and only these, are pains? (ii) The question of commensurability: Are all pleasures and pains rankable on a single, quantitative hedonic scale? I argue that our intuitions draw us in opposing directions: On the one hand, pleasures and pains seem unified and commensurable; on the other hand, they do not. I further argue that neither intuition can be abandoned, and examine three different paths to reconciliation. The first two are response theory and split experience theory. Both of these, I argue, are unsuccessful. A third path, however—which I label “dimensionalism” —succeeds. Dimensionalism is the theory that pleasure and pain have the ontological status as opposite sides of a hedonic dimension along which experiences vary. This view has earlier been suggested by C. D. Broad, Karl Duncker, Shelly Kagan, and John Searle, but it has not been worked out in detail. In this paper I work out the dimensionalist view in some detail, defend it, and explain how it solves the problem of the unity and commensurability of pleasures and pains.