We have selected from the current theory of defense mechanisms those concepts which seemed most valid to us from the standpoints of child observation and clinical experience. The hypothesis that defense mechanisms are cognitive structures is presented and applied to the development of individual defense mechanisms. We have also taken up the concept of intersystemic suborganizations, and discussed the role defense mechanisms play in these structures. Lastly, we have noted that in the course of successful psychoanalysis, when the suborganizations are exposed and the conflicts worked through, the defensive operations tend to lose their primitive character and to take on the more refined and adaptive qualities of the cognitive functions of which they are examples.