The study of trade union structure has rested upon the taxonomic differentiation of unions into structural types. Descriptively these taxonomies reflect the origins and historical development of many unions rather than the structural forms currently exhibited. Analytically, their usage places emphasis upon stasis within trade union structure to the relative exclusion of the dynamics inherent within trade union development. Waddington (1987) examined structural dynamics by means of the study of trade union structural events: formations, mergers, dissolutions and breakaways. This paper focuses upon merger activity, with the twofold objective of elaborating a framework within which structural dynamics can be analysed, and establishing those environmental factors which influence merger activity.Trade union mergers are one of the means whereby trade union structure develops. Two divergent approaches have been adopted for the study of trade union mergers both of which are limited in allowing for the establishment of an explanation of trade union merger activity. Buchanan (1974 and presents data illustrating the aggregate pattern of merger activity. Factors explaining this pattern are advanced by reference to the coincidence between peaks in merger activity and environmental indicators. This coincidence does not, of course, necessarily imply causality, nor does Buchanan's explanation incorporate an analysis of the changing complex of inter-relationships existing among the factors that influence merger activity. For example, the historical developments in the institutional form of trade unionism imply a changing character to the merger process and, hence, brings into question the uniformity of the impact of the environmental factors. Additionally, no reference is made to other trade union structural events and how these might impinge upon merger activity.A second approach relies upon the analysis of particular merger case studies to identify those factors that influence mergers, and to differentiate between types of merger (for example, Hughes, 1967:15-23 and Undy et al., 1981:167-219). In order to move towards an overall explanation of mergers such an approach relies on the assumption that the selected case studies are indicative of all mergers. The case study approach also limits any assessment *Lecturer, Industrial Relations Unit, University of Keele. 410 British Journal of Industrial Relations of the historical development of trade union structure and the changing nature of the processes underlying this development.While both these approaches provide useful material for the study of trade union mergers they are analytically distinct. The purpose of this paper is to provide an explanatory framework for the study of trade union mergers. My earlier research shows that such a framework provides a basis to the understanding of trade union structural dynamics and the selection of merger case studies by means of which internal union political factors influencing structural development can be historically located. The centr...