This paper analyzes individual decision making under risk. It is assumed that an individualndoes not have a preference relation on the set of risky lotteries. Instead, an individual possesses a probability measure that captures the likelihood of one lottery being chosen over the other. Choice probabilities have a stochastic utility representation if they can benwritten as a non-decreasing function of the difference in expected utilities of the lotteries.nChoice probabilities admit a stochastic utility representation if and only if they are complete, strongly transitive, continuous, independent of common consequences andninterchangeable. Axioms of stochastic utility are consistent with systematic violations of betweenness and a common ratio effect but not with a common consequence effect. Special cases of stochastic utility include the Fechner model of random errors, Luce choice model and a tremble model of Harless and Camerer (1994).