The following work is devoted to an account of the characteristics of crowds. Organised crowds have always played an important part in the life of peoples, but this part has never been of such moment as at present. The substitution of the unconscious action of crowds for the conscious activity of individuals is one of the principal characteristics of the present age.Crowds, doubtless, are always unconscious, but this very unconsciousness is perhaps one of the secrets of their strength. In the natural world beings exclusively governed by instinct accomplish acts whose marvellous complexity astounds us. Reason is an attribute of humanity of too recent date and still too imperfect to reveal to us the laws of the unconscious, and still more to take its place. The part played by the unconscious in all our acts is immense, and that played by reason very small. The unconscious acts like a force still unknown.