There are several theories of trade unions discussed in the document. Sidney Webb's theory of industrial democracy views trade unions as an extension of democracy from the political sphere to the industrial sphere through collective bargaining. Karl Marx's revolutionary theory considers trade unionism a form of class struggle that will lead to the transition from capitalism to democratic socialism. Hoxie's theory of business unionism and Perlman's theory of scarcity consciousness also view trade unions as serving workers' economic self-interest through collective action.