The clash between two pioneers: its effect on the development of group psychotherapy in the United States and Canada - This paper traces the development of group psychotherapy in the USA and Canada. It began with the pioneering efforts of two creative men, S.R. Slavson and J.L. Moreno. Early on, rivalry between them caused the movement to split in two. Parallel organizations were created under their influence. The differences between each one’s philosophy of how an organization should function were reflected in the subsequent development of the two interdisciplinary professional societies that they founded. In addition, socio-economic and cultural aspects of the region have had an impact on the function of these societies as well as on the practice of group psychotherapy in this region.