This chapter is concerned with the constitutional position of trade unions in the United Kingdom. Modern trade unionism emerged from the industrial revolution to protect the interests of workers, and from the late eighteenth century increased in membership and influence, both of which peaked in the 1970s. There were then about 13 million trade unionists, representing some 60 per cent of the working population. But trade union influence extended more widely, touching the lives of almost every worker in the country, either through the coverage of collective agreements negotiated with employers or employers’ associations, or through legislation on a wide range of matters that trade unions had persuaded governments to introduce.