 The first two sections of this entry seek mainly to explicate theory theory and its possible variations. Section 1 describes the core commitments of theory theory, and the traditional reasons offered for believing in it—noting the assumptions that theory theorists traditionally embrace about the primary function of folk psychology, and the various, heavier or lighter, options they have for construing its ontological commitments. Section 2 details three varieties of theory theory, looking at where they disagree about such things as: what kind of theory folk psychology is; whether it is concept-based or model-based; whether and how it is acquired; and whether and how it develops.