The "borderline" personality disorders are examined from the descriptive, structural, and genetic-dynamic viewpoints. It is suggested that they have in common: (i) symptomatic constellations, such as diffuse anxiety, special forms of polysymptomatic neuroses, and "prepsychotic" and "lower level" character pathology; (ii) certain defensive constellations of the ego, namely, a combination of nonspecific manifestations of ego weakness and a shift toward primary-process thinking on the one hand, and specific primitive defense mechanisms (splitting, primitive idealization, early forms of projection, denial, omnipotence), on the other; (iii) a particular pathology of internalized object relations; and (iv) characteristic instinctual vicissitudes, namely, a particular pathological condensation of pregenital and genital aims under the overriding influence of pregenital aggressive needs. These various aspects of borderline personality organization and their mutual relationships are briefly examined.