On 31 July 2017 CNA conducted an internal workshop of experts to discuss the implications of the 4 plus 1 challenges on maritime security cooperation (MSC). The outcome of this workshop fed directly into one of several background briefings to be delivered to the 2017 Maritime Security Cooperation Working Group, which carries the theme of transregional challenges and best practices in planning and executing MSC. This volume contains five short essays that served as a basis for that discussion and which will help in outlining the major issues surrounding each of the 4 plus 1 challenges and their implications for MSC. The essays are not research based; nor are they comprehensive treatments of the issues. Rather, each is designed to prompt thought and discussion about the role of U.S. MSC in the context of the 4 plus 1 framework.