This chapter looks at the major published sources relating to teacher union activity from three broad perspectives which are characterized as: the Historical Approach, the Sociology of the Professions, and Pressure Group Theory. Thompson, Tropp, Roy and Gosden each deal with the historical development of teachers' unionism in parallel with the growth and development of state provision of education. Tropp, Roy and Gosden all promote the impression of organized teachers acting consistently in a non-political, non-class based movement towards professionalism. This chapter argues that the concept of professionalism is an extraordinarily complex one and its different meanings must be located within a specific historical context. Histories of teacher unionism have created an image of consistent movement towards professionalism, an image which at once reflects and supports the 'natural history' approach to the professions. The Union combines the activities and services of a professional organization and a trade union.