This article reviews the broad political economy literature on transatlantic security, while noting applications to the current challenges ranging from burden-sharing to enlargement to NATO/EU relations, all in the context of renewed state-centric threats to allies emanating from Russia in particular. Thinking about the policy practicalities of transatlantic security has interested scholars and practitioners since the early days of the post-WWII international order. The “O” in NATO emerged as an international “organization” in many ways to address the economic challenges posed by a defense investment imbalance, wherein the US assumed the largest share. This collective action problem and the concomitant burden-sharing challenges merit yet another consideration given a deteriorating security environment in Europe and globally. We discuss a variety of influences that shape the ability of the transatlantic community to invest in defense, while also highlighting the role of recent empirical developments, notably, NATO’s prospective enlargement and the EU’s policy initiatives in the defense field. The discussion lays groundwork for a broader approach to transatlantic security, that is, beyond the canonical understanding of the collective action problem, and stresses a policy perspective that encourages thinking about levers by which burden-sharing arrangements can be adjusted to deal with unfavourable dynamics of defense investment.