What factors explain the Australian trade union merger wave between 1991 and 1994? Existing explanations largely attribute it to the pro –amalgamation policy of the Australian Council of Trade Unions (ACTU)and other union leaders,and to declining union membership and decentralised bargaining. This paper reviews discussion of the causes of mergers and publicly available evidence upon them. It concludes that current explanations of the merger wave are an over–simplification. The effects of ACTU leadership, official union policy,and members' views are complex and not uniform and require more disaggregated analysis. Also,there has been a tendency to overstate the importance of membership decline and decentralised bargaining and to over – look other environmental factors such as changing occupational structure. The paper cautions against the assumption that variables influencing ACTU policy also shape affiate actions.