Final State Interactions in $ε'/ε$

Strong final state interaction (FSI) effects can play a central role in the Standard Model prediction of weak $K\to 2\pi$ matrix elements. Here, I discuss how FSI's affect the direct CP violation parameter $\vepp / \vep$ by solving the Omn\`es problem for the necessary $K\to 2\pi$ amplitudes. The main results of this analysis have been reported in a previous paper [1] and are further discussed in Refs. [2,3]. The resulting Standard Model prediction ${Re} (\vepp / \vep) = (15\pm 5)\times 10^{-4}$ is in good agreement with the present experimental world average.