This chapter presents a study of the magnetic properties and magnetic interaction mechanisms in a series of thiazyl radicals, which are of particular interest in the field of molecular magnetism. The importance of the accurate knowledge of the spin density distribution to understand the mechanisms of the magnetic interactions is discussed. In the Anderson formalism for the magnetic exchange interaction, the kinetic term is usually dominant and the magnetic interaction is antiferromagnetic. It is found that when the net overlap between the natural magnetic orbitals is zero, the kinetic term vanishes and the magnetic interaction is ferromagnetic. It is observed that within the unrestricted Hartree-Fock formalism, the spin polarization phenomenon arises from the different exchange interaction between the unpaired electron in the singly occupied molecular orbital (SOMO) with the spin up (α) and spin down (β) electrons in the fully occupied molecular orbitals (FOMO). The spin densities in the dithiadiazolyl ring are also elaborated in the chapter.