Sándor Ferenczi did not work with groups, yet his thinking and practice can be conceived as a major influence on the origins and development of group analysis and as a seminal source of ideas for its further development today. In his approach to psychoanalysis, social facts have a bearing on psychological facts, and vice versa. This implies a constant interchange and mutual influence between individual and collective processes, inner and outer, psychological and social. This is one of the basic tenets of group analysis, which requires that social facts be given as much attention as the psychological. Ferenczi’s major contributions to the emergence of group analysis are his conceptual and technical revolutionary innovations, centred on the essential unity and mutual interchange between transference and countertransference. His emphasis on the fundamental importance of actual relations with other significant persons, such as the primary caregivers, the family, the analyst and society, and his description of the dynamic interplay of transference and countertransference anticipated the theoretical developments of group analysis. He introduced the holistic concept of the unity of transference and countertransference and of the family and the child. This implied an underlying but yet unformulated field theory, which is basic for group analysis. The essential unity and mutual interchange of ‘inner’ and ‘outer’ may be accounted for by Ferenczi’s concept of an originary undifferentiated state of mind, called ‘Thalassal’, from which all other mental states, experiences, perceptions and thoughts evolve, and which remains present but unseen, underlying the more differentiated states. This is the psychological basis for Foulkes’ conception of the matrix and Pichon-Rivière’s theory of the link (bond). Ferenczi’s memory was long repressed by the psychoanalytic world, but now that it has been recovered, his contributions may provide many of the missing pieces of group-analytic theory and practice and build the much-needed bridges between psychoanalysis and group analysis.