This chapter engages in an exegesis on the defence power as set out in the Commonwealth (Australian) Constitution. It highlights the settled features of the defence power, its expansion and contraction by reference to the prevailing wartime or peacetime circumstances. It provides a succinct analysis of the decisions of the High Court of Australia that define the parameters of the power. Special attention is given to the Communist Party Case and the significance of the case for the rule of law. The case of Thomas v. Mowbray is analysed for its impact on the jurisprudence pertaining to the defence power. Attention is given to the application of the proportionality principle and the limits of the defence power.