The question as to whether simulation or theory form the basis of our Theory of Mind (ToM) abilities remains open and important. Since simulation theory (ST) has developed in opposition to theory-theory (TT), we can understand ST by considering TT. This chapter sketches some initial motivation for considering ST. After that, it examines the two most important variants of each of the two competing theoretical and simulational accounts of ToM. Then, the chapter analyzes the scientific variant of TT, under which ToM is theoretically based and the theory used is akin to a scientific theory. This account is the mainstream one in psychology and is the one favoured by Saxe. The chapter also examines the innate variant of TT, which also claims that the ToM is theoretically based, but denies that the theory is like a scientific theory. On this modular account, which is also known as ToM-Mechanism (ToMM), humans are born with the theory that underlies their ToM.