The concept of the social unconscious has not been studied with respect to the work of S. H. Foulkes taken as a whole. Apart from the abstract survey of his ideas implicit in the Introduction to The Social Unconscious in Persons, Groups and Societies: Volume 1: Mainly Theory (Hopper & Weinberg, 2011), the development of the concept of the social unconscious from its Jungian and Freudian origins to Foulkes’s mature group analytic theory has been neglected. No serious attempts have been made to link the concept of the social unconscious to the “matrix”, which Foulkes called the “corner-stone of our working theory” (Foulkes & Anthony, 1957, p. 217). Accordingly, the purpose of this chapter is to support with citations from the work of Foulkes the argument outlined by Hopper and Weinberg (2016) in their Introduction to Volume 2: Mainly Foundation Matrices, that the main elements of his more mature theory of the social unconscious include sociality and socialisation, relationality, transpersonality, transgenerationality, and collectivity, ultimately subsumed by the matrix. This field theory of the social unconscious and the matrix was a product of Foulkes’s continuous and persistent thinking about these elements and their interconnections over a period of four decades.