Despite the long history and continuing importance of peak union bodies in Australian industrial relations, there have been very few attempts to analyze, let alone theorize about, their development. This omission in the literature is particu larly striking in relation to the origins of such bodies. Those rare treatments that do examine the formation of peak union bodies are unsatisfactory; most either assume that such forms of unity are inevitable or point to external causes in a rather mechanistic way. We propose a model of peak union formation that com bines internal and external processes, emphasizing the connection between them. We argue that for any group of unions to form a peak body, a state of internal equilibrium must exist between the unions concerned, and that these unions must also be presented with a clear external threat or opportunity. The model is used here to explain the origins of one of the oldest continuous local peak bodies in Australia, the Barrier Industrial Council, but we suggest that the model has a general applicability.