To develop one's own separate identity and sense of personal authority is a particularly difficult task within a twinship relationship. Twinship creates the additional obstacle of separating from a need-satisfying sibling before progressing to the struggle for separation from mother. This paper examines these difficulties within the metaphor of twinship, linking them to the development of identical twins, object relationships and psychotherapeutic practice. Twinship is discussed as a schizoid phenomenon, with reference to reality, narcissism, relatedness, mirroring, dependency needs, boundaries and space.