This article seeks to contribute to a transatlantic discussion by examining the theoretical and policy claims for and against NATO expansion that have dominated discussions in the United States. American academics have been almost unanimous in opposing NATO expansion, but the arguments they have put forward have often been contradictory. The proponents of NATO expansion, on the other hand, have not been clear in their arguments, and have tended to stress deterrence over reassurance. It is suggested here that there are indeed good reasons to expand NATO. Central European states have legitimate security concerns and so does Russia. However, an expanded NATO's value rests in its ability to reassure not only its new members but Russia and other countries in the region. If its focus remains primarily to coordinate military plans for deterring attacks on its members alone, its expansion will most likely exacerbate security relations in Europe.