The book examines NATO’s northern enlargement from the perspective of both the Alliance as well as the new members, Finland and Sweden. The key aim is to provide a comprehensive and balanced account of the historical background, the actual decision-making as well as the already present and possible future consequences of NATO’s enlargement to the north. It contributes to the scholarly literature applying theoretical perspectives on NATO enlargement policy by arguing that the enlargement to the north was compatible with both realist and partly constructivist explanations. The book offers in-depth case-specific accounts of the policy processes which took Finland and Sweden to members of NATO, including the difficult accession negotiations with Turkey. Finally, the book analyses NATO’s new northern flank and provides an updated policy-relevant map of a new strategic landscape.