The embryologic development of the CNS is a complex process involving migration, induction, specialization, differentiation, and remodeling. This process occurs in a system of co-developmental cooperation in coordination with the entire body. Malformations of the CNS are frequently associated with malformations of these systemic organs and major heterotopias. While of major interest to neuroscientists, these malformations of the CNS are covered in extensive detail in other publications. This chapter will address the hamartomas and choristomas that enter into the consideration of mass lesions of CNS derivation that either may be confused with neoplasms or are neoplastic themselves.