This chapter is concerned with group performance on decision-making tasks. Collective decision making, like collective judgment (Chapter 5), is a cooperatively performed cognitive activity in which it is difficult for members to demonstrate conclusively to one another that a proposed response (decision alternative) is in fact best. This difficulty arises primarily because the conceptual systems within which such decisions are made are rather imprecise about how the information available to decision makers should be interpreted and combined (cf. Chapter 2, pp. 48-49). Indeed, those systems may be so ill-defined as to suggest only vague, and perhaps conflicting, evaluative criteria, thus encouraging disagreements among members about such basic matters as the objectives to be achieved in making their decision. A parole board, for example, may struggle with how to weight retributive versus rehabilitative goals when making early prison release decisions. And in a particular case, they may argue at length about whether or not an inmate’s belated, and seemingly self-serving, admission of guilt should be factored into their ruling. Thus, although it is fundamentally a cooperative activity, group decision making can sometimes be an occasion for substantial interpersonal tension and conflict.