Diplomatic, military and economic relations between Australia and the United States have been close for a century. The security relationship on which this chapter focuses was cemented during the Second World War and formalised by the ANZUS Treaty in 1951; it is central to Australia’s strategic calculations. Despite uncertainty about the future of the alliance during the early stages of Donald Trump’s presidency, Australian political leaders are committed to it while conscious of Australia’s economic dependence on trade with China, the United States’ strategic competitor. Given the parallel interests and shared values of the United States and Australia the prospects for the relationship going forward are positive, though the advent of the Trump administration demonstrates that leaders’ personalities cannot be discounted. Above all, the rise of China and Australia’s abiding sense of insecurity mean that the United States and Australia are likely to remain close allies for the foreseeable future.