abstract: In this talk I will present a model for Knightian uncertainty that does not carry a probabilistic structure. In this framework, I will prove the equivalence of economic viability of asset prices and absence of arbitrage. In particular, these extend of the results of Kreps 1981, and provide a modified version of the Fundamental Theorem of Asset Pricing through sub-linear pricing measures. Different versions of the Efficient Market Hypothesis will also be discussed. This is joint work with Matteo Burzoni of University of Milano and Frank Riedel of University of Bielefeld.