We may distinguish at least the following live sufficiently inde­pendent categories of vibratory processes differing in their nature: free vibrations, i.e., vibrations which are performed by a mecha­nical system having no energy supply from outside if the system is disturbed from its position of equilibrium and then released; critical states of rotating shafts and rotors which consist in a sudden increase in the deflections of their axes at definite speeds of rotation (or in definite ranges of speeds);forced vibrations which result when the mechanical system is acted on by fluctuating external forces (driving forces);parametric vibrations caused by periodic variations of the para­ meters of a system (for example, its stiffness);self-excited vibrations, i.e., vibratory processes which are main­tained by constant sources of energy of a non-vibratory nature.Each of these categories of vibratory processes is discussed in the appropriate chapter.