Searches for Physics beyond the Standard Model with Kaons at NA48 and NA62 at CERN

The ratio $R_K = \Gamma(K^{\pm}\to e^{\pm}\nu(\gamma)) / \Gamma(K^{\pm}\to\mu^{\pm}\nu(\gamma))$ provides a powerful probe of the structure of weak interactions. It is calculated with very high precision within the Standard Model, but corrections due to the presence of New Physics could be in a few percent range. Development of NA48/NA62 method based on test data samples of 2003-04, and the status of analysis based on a dedicated 2007 run are discussed. In addition, a proposal to measure the ultra rare decay $K^+\to\pi^+\nu\bar\nu$ at the CERN SPS is presented.