The process of motivation of an activity and of particular actions is analyzed. Motivational process is regarded as an internal psychological act concerned with development of a drive for an act or an activity, and with maintenance of this drive in the process of realization of the activity. This requires to distinguish the notions of "motive" and "drive". It is recommended to use the term "motive" as a denomination of stable psychological formations which underly the choice of goals, acts, means and ways of acting, and which determine the partiality of behavior. "Drive" is understood then as an immediate cause found at the beginning of an activity. An attempt is made to show that voluntary motivational process is a mechanism of volitional behavior.