This paper proposes that the unconscious undergoes first a progressive phase and then a regressive one and that these two phases correlate with the growth and decline of the biological organism. Biological growth proceeds in a straightforward, progressive line until adulthood is reached at about the age of 20. A similar progresssive movement of the unconscious underlies psychic development during that period. After this psychobiological "ascent" of the libido follows a "descent" in line with the natural principle of exhalation and inhalation, diastole and systole, day and night. The reversal of the flow of the unconscious into a regressive one is inaugurated in a dramatic manner by the Jugendkrise. After this crisis of one's youth follows a deep and sustained regression of the libido, which retraces the path of postnatal and prenatal development, step by step, along the line which it had pursued during its forward course. This Tiefenregression leads in the second phase of life to a revival of the psychobiological imprints of man's entire intrauterine development, the symbolic projections of which are to be found in man's dreams, art and religion. The ultimate aim of this regressive and transpersonal movement is to establish the unity of man and nature, conscious and unconscious, ego and self.