Conjoined twins are an extremely rare phenomenon occurring in roughly up to 1.5 per 100,000 births (Mutchinick et al. 2011; Martínez-Frías et al. 2009). The precise etiology and mechanism are unclear, but some authors believe that conjoined twins result from a failure of development of primitive structures at later stages of embryonic development, particularly during Carnegie stage 6 (days 12–15), or the primitive streak stage of human development (Levin et al. 1996; Kaufman 2004a, b; Sadler 2010). This chapter summarize the limited data regarding the central nervous system malformations that may beobserved in association with conjoined twins.