This text reviews a recent approach to modeling "radically uncertain"

behavior in strategic interactions. By rigorously rooting the approach

in decision theory, we provide a foundation for applications of Knightian uncertainty in mechanism design, principal agent and moral hazard models. We discuss critical assessments and provide alternative

interpretations of the new equilibria in terms of equilibrium in beliefs,

and as a boundedly rational equilibrium in the sense of a population

equilibrium. We also discuss the purification of equilibria in the spirit

of Harsanyi.