A description is given of the apparatus which represents realization of the well-known principle of cancellation of the Doppler effect in two-photon transitions. A brief review is given of the applications of the Doppler-free two-photon spectroscopy in the measurements of the fine and hyperfme structure of highly excited states, isotopic shifts, etc. Collisional broadening and spectral shift in sodium vapor mixed with various buffer gases is considered. A detailed discussion is given of the application of the Doppler-free two-photon spectroscopy to studies of transient optical processes. The method is extended to the case of many-photon transitions. A description is given of the apparatus suitable for the observation of Doppler-free three-photon transitions. Advantages and shortcomings of the new and existing methods are discussed.