This paper presents an axiomatic foundation for the maxmin expected utility decision rule, which extends classical expected utility theory by allowing for a set of possible priors rather than a single prior. The authors characterize preference relations over acts that can be represented by the functional J(f) = min{∫u∘f dP | P ∈ C}, where f is an act, u is a von-Neumann-Morgenstern utility over outcomes, and C is a closed and convex set of finitely additive probability measures on the states of nature. The paper introduces a new condition, uncertainty aversion, which is a weakening of the classical independence axiom. It also defines a concept of independence in the case of a non-unique prior. The authors show that if the preference relation is non-degenerate, the convex set of priors C is uniquely determined. The paper also discusses the relationship between maxmin expected utility and Wald's minimax criterion, and how it relates to personalistic probability. The main result is that a preference relation over acts satisfies certain axioms if and only if it can be represented by the functional J(f) = min{∫u∘f dP | P ∈ C}. The paper also extends the result to a broader class of acts and discusses the uniqueness of the utility function and the set of priors under certain assumptions.This paper presents an axiomatic foundation for the maxmin expected utility decision rule, which extends classical expected utility theory by allowing for a set of possible priors rather than a single prior. The authors characterize preference relations over acts that can be represented by the functional J(f) = min{∫u∘f dP | P ∈ C}, where f is an act, u is a von-Neumann-Morgenstern utility over outcomes, and C is a closed and convex set of finitely additive probability measures on the states of nature. The paper introduces a new condition, uncertainty aversion, which is a weakening of the classical independence axiom. It also defines a concept of independence in the case of a non-unique prior. The authors show that if the preference relation is non-degenerate, the convex set of priors C is uniquely determined. The paper also discusses the relationship between maxmin expected utility and Wald's minimax criterion, and how it relates to personalistic probability. The main result is that a preference relation over acts satisfies certain axioms if and only if it can be represented by the functional J(f) = min{∫u∘f dP | P ∈ C}. The paper also extends the result to a broader class of acts and discusses the uniqueness of the utility function and the set of priors under certain assumptions.