The optical properties of sandwiched semiconductor structures consisting of a sequence A-B-A-B... of atomic smooth layers can be modelled intentionally by adjusting the proper layer thicknesses in an epitaxial growth process. For period lengths of a few hundred angströms quantum confinement effects of the wavefunctions of carriers (electrons and holes) lead to new phenomena like formation of discrete energy levels in quantum wells, or of new energy bands (“minibands”) in superlattices. Absolute (transmittance, reflectance, absorbance) and modulation infrared spectroscopy (photo-, electro-, piezoreflectance, differential photoreflectance) have been developed for measuring the change of optical absorption as a function of the incident light, and optical or d.c. electrical fields. In our work wide-gap (GaAs/GaAlAs, ZnTe/CdTe) and narrow-gap (PbTe/PbEuTe) semiconductor superlattices are investigated with various layer thicknesses. It has been proved that the optical response (located in the near- and midinfrared for those systems) depends strongly on the two-dimensional behaviour of a confined electron gas (2DEG). FT-IR transmittance spectra are presented for the 2DEG absorption in a PbTe/PbEuTe multi-quantum well (MQW), photo- and piezoreflectance spectra for GaAs/GaAlAs-MQWs, and far infrared reflectance spectra of confined lattice vibrations in a short period CdTe/ZnTe superlattice. The transmittance spectra are also calculated and compared to the measurements.