A possible approach for finding physics beyond the Standard Model of Particle Physics are high-precision tests of the Standard Model at low energies to search for possible deviations from Standard Model predictions. For any quantity involving QCD, the only known approach for an ab-initio treatment at low energies is Lattice QCD. In this talk I will discuss two quantities that can be used to test the Standard Model, namely leptonic Kaon decays, where further improvements of the Standard Model prediction require the calculation of isospin breaking corrections, and the rare decay of a Hyperon $\Sigma^+\to p\mu^+\mu^-$.