In this chapter we will present the simplest calculations, the so-called tree-level calculations that do not yet require a deep knowledge of quantum field theory like renormalization and renormalization group. This step will give us leading formulas for the determination of various parameters of the SM like the Fermi constantand the parameters of the CKM matrix, which will be introduced and discussed in detail. These formulas will also give us a transparent picture of the leading decays ofmesons and leptons and will allow approximate determination of the parameters in question. Corrections to these leading expressions will be calculated or simply listed in later chapters. In this chapter we will also find out that in the case of meson decays some entries in the decay amplitudes cannot be calculated in perturbation theory. These are hadronicmatrix elements of quark currents and of four-quark operators. Their calculation will be done in other chapters.