Various publications have focussed attention on the building trade unions, usually with the aim of presenting an account of the history of an individual union. In most of them it has been assumed that one union for the building industry was the real and realisable objective of many of the actors in that history. The object of this thesis is to assess the validity of that assumption in the light of discussions on structural change within and between the unions concerned. The work takes the form of a historical account spanning the years of the twentieth century. It is based on surviving documentation and interviews with former officials of the unions concerned. Firstly it is argued that changes in the labour process were a necessary pre—requisite for changes in trade union structure. The labour process is defined according to its specific social form, that is as a capitalist process of production and emphasis is placed on capital formation, on government policy, on the level of technology and on the division of labour within the construction industry, as factors which explain the long survival of a craft form of trade union organisation. Attention is directed to changes in the form of engagement of labour, to the emergence of labour—only sub—contracting, and its significance for trade union organisation in construction. Secondly it is suggested that there is a close relationship between trade union government and trade union structure. Adjustments to the shape and size of trade unions are used by their leaders to foster their own authority and status within the emergent organisation. It is concluded that these factors were more important in motivating changes than any commitment to a particular form of trade union organisation.