Bion and Jung developed their respective proto-mental and psychoid concepts contemporaneously and in the face of certain similar influences, including WW1 and its aftermath, as well as concurrent advances in medical, psychoanalytic and philosophic thinking. Each concept contemplates a deeply unconscious and unknowable area, in which body and mind are undifferentiated and from which emerge manifestations that are as much physical as psychic. However, while the two concepts bear some similarities, they also differ in certain fundamental respects. This paper traces the overlapping history of the two concepts, noting the effects of the various influences on both authors and on their development, and enquiring into the extent to which they may have been similarly influenced by thinkers, such as Bergson, as well as the extent to which Bion may have been influenced by Jung and his ideas. A comparison of the two concepts offers both an opportunity for clarifying and refining an understanding of each one, and an opportunity for reviewing the way that the psychoanalytic field was evolving during the first half of the twentieth century in differing but complementary directions.