This chapter evaluates the Atlantic Alliance’s significance for Italian foreign and security policy and Italy’s role in transatlantic security in the 21st century. It focuses on Italy’s role in North Atlantic Treaty Organization (NATO) in the aftermath of the terrorist attacks of September 11, 2001, and during the Global War on Terror (GWOT). The chapter examines the impact of the 2008 financial crisis on Italy’s role within the Alliance. It discusses Italy’s perceptions of NATO’s policy toward Russia. The chapter describes the Donald Trump administration’s impact on Italy’s current and prospective role within the Alliance. The chapter provides a reappraisal of Italian perceptions of, and attitudes to, the Alliance in the 21st century. At the beginning of the new millennium, the George Bush administration’s proclamation of the GWOT added an element of complexity to Italy’s relationship with the Alliance, presenting Italian decision-makers with a novel and potentially disrupting scenario.