This thesis examines the political activity of the Communist party of Australia (CPA) in the trade union movement between 1945 and 1960. It represents the first systematic scholarly analysis of this activity. The historiography of the CPA has generally focused on the industrial activity of CPA trade union members and has neglected this dimension. The thesis draws on CPA newspapers and journals, Congress resolutions, the publications of Communist-led unions and numerous secondary sources to argue that explicit political activity in the unions was often central to CPA activity in this period. The approach was consistent with orthodox Marxism, which regarded trade unions as a preparatory school for increasing the political consciousness of workers as a prelude to an anti-capitalist revolution. This political trade unionism distinguished the CPA from other political currents in the labour movement which may have accepted its militant unionism, but not its advocacy of political trade unionism. This thesis examines three areas of this political unionism: the attempt to build trade union support for the peace movement, the attitudes towards the post-war mass immigration programme and the emerging Aboriginal civil rights movement.