We dene the distance between two information structures as the largest possible dierence in the value across all zero-sum games. We provide a tractable characterization of the distance, as the minimal distance between 2 polytopes. We use it to show various results about the relation between games and single-agent problems, the value of additional information, informational substitutes, complements, etc. We show that approximate knowledge is similar to approximate common knowledge with respect to the value-based distance. Nevertheless, contrary to the weak topology, the value-based distance does not have a compact completion: there exists a sequence of information structures, where players acquire more and more information, and $\epsilon$ > 0 such that any two elements of the sequence have distance at least $\epsilon$. This result answers by the negative the second (and last unsolved) of the three problems posed by J.F. Mertens in his paper Repeated Games", ICM 1986.