Recent results on the far infrared and microwave spectroscopy of PbTiO_3 -PbZrO_3 system (including PLZT), BaTiO_3 -SrTiO_3 system and PbSc_1/2Ta_1/2O_3 (B-site ordered and disordered) single crystals, ceramics and thin films is reviewed and discussed emphasising also the low-frequency dielectric properties. Generally the soft phonon mode does not soften completely and does not account for the total value of the low-frequency dielectric permittivity. Therefore additional microwave and/or strong radio-frequency dispersion appears, caused by the disorder and/or strong lattice anharmonicities. The thin films show, as a rule, much broader phonon spectra (higher effective soft-mode damping) and smaller low-frequency permittivity compared to the bulk samples. The differences between bulk and htin film behaviour are more pronounced in the (Ba,Sr)TiO_3 system than in the (Pb,La)(Zr,Ti)O_3 system.
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