Abstract This work is devoted to high order harmonic generation (HOHG) on steep plasma density gradients during the interaction of relativistic femtosecond laser pulses with solid density plasma. A qualitative change to the specular reflected HOHG mechanism has been observed when the intensity of the p-polarized excitation pulse increases beyond the relativistic threshold. It has been experimentally verified that harmonic generation takes place in the case of the s-polarized relativistic excitation beam. A wave mixing experiment has been carried out. The behaviour of HOHG efficiency as a function of the scale length is measured. The results of the first three experiments have been explained with the help of the “oscillating mirror” model and the “resonance” mechanism of HOHG which are presented in the first chapter. The recorded dependence of HOHG efficiency on the scale length is qualitatively in good agreement with the predictions of the particle in cell (PIC) simulations.