This article examines the Commonwealth’s conception of political undesirability between the wars as embodied in policies governing the boundary of citizenship and the movement of people. The article focuses on the role of the Commonwealth Investigation Branch (CIB) in the formulation of criteria of political undesirability and the development and operation of mechanisms for the application of such criteria to these policies. It is argued that following the Great War the Commonwealth’s conception of the desirable citizen expanded to include characteristics relating to political ideas and behaviour. An analysis of the Commonwealth’s policies of naturalisation, immigration, deportation, the compulsory registration of aliens, and passports provides insights into the official conception of the Australian citizenry. Further, the article argues that the CIB’s role in these policies occupied a substantial proportion of its time, and that its role was significant for the application of political criteria to these policies.