This chapter discusses the general property of IV-VI semiconductor materials. The binaries PbS, PbTe, and PbSe (ordered by their band gap equivalent wavelengths), and typical mixed crystals of this material family like the ternaries PbSnTe, PbSnSe, PbCdSe, PbSSe cover a band gap range from some 50meV to 500meV, with a corresponding wavelength range of roughly 3μm to 30μm. The chapter overviews the relation of the band gap of some mixed crystals on the lattice periodicity length. The band structure and constant energy surfaces of PbTe are shown. Band gap energy versus lattice constant for IV-VI materials and band offsets for conduction and valence bands at PbEuSe/PbSe interfaces are illustrated. The theoretical density of states of a  grown PbEuTe/PbTe superlattice in comparison to the bulk material density of states (DOS) is shown. The comparison of the spectra of the absorption coefficient and of the refractive index of a PbEuTe/PbTe multi-quantum well has been shown.