Siamese twins share some body surface, and may also share organs and/or limbs, as well as having areas of joint sensation and movement. Apart from issues relating to the nature-nurture debate, psychology has paid little attention to the phenomena of Siamese twinning. This paper discusses the phenomenal experience of body boundaries by Siamese twins in relation to issues of self and identity. It is argued that aspects of individual embodiment provide an ambiguous body boundary for this group. Specifically, it is argued that phenomena such as areas of common tactile sensation (overlapping ‘sensory rims’) and the ability to move shared limbs (divided ‘authorship of action’), fosters an experience of body as sentiently and spatially extended into their sibling.