The paper develops a model of trade union behaviour based on the concept of the viable bargaining unit. Viability rests on five conditions: membership level, service level, membership participation, employer recognition, and facilities. Unions are seen as portfolios of viable and inviable bargaining units. Six propositions are derived, concerning union scale, growth, the impact of statutory recognition provisions, the emergence of conglomerate unions, governance structures, and relations with employers. Employer dependence is central, and a simple game-theoretic approach is used to discuss employer co-operation. Viability at the union level is achieved by portfolio diversification and employer co-operation.