Far-infrared spectroscopy was one of the important experimental techniques for the investigation of classical superconductors. Not long after the clear prediction of an excitation energy gap by the BCS theory, the gaps of various superconductors could be found spectroscopically. At that time the experiments were extremely difficult, since the gap frequencies occurred in the very far infrared and because the spectroscopic techniques were not as far developed as today. Because gap frequencies (omega) g and critical temperature Tc of superconductors are directly interrelated, one should expect the gaps of the high-Tc cuprates to occur at higher frequencies in the far infrared, thus being easier to access. It turned out, however, that this is not the case by reasons which are described later. At the moment, no completely clear evidence for the positions of the gaps exists in the case of clean, undoped high-Tc superconductors, while clear evidence was found in the case of iron- doped YBa2Cu3O7-(delta ). This paper describes mainly the problems concerning the spectroscopic methods, while the superconducting properties are only briefly sketched.