There has been a gradual move towards recognising more diverse security issues as posing security threats to more diverse actors in broader frontiers. While the multidimensionality of security is now widely acknowledged in the discourse of security, its impacts on and challenges to international law are yet to be fully examined. Particularly, the expanded conception of security has posed challenges to the UN collective security system. This article considers the challenges posed to collective security, with respect to four different objects of security: national security, international security, human security, and regime security. It discusses the limits of collective security in effectively responding to the expanded conception of security within the existing framework of international law, and revisits alternative security approaches, evaluating their potential to complement collective security in dealing with diverse security objects and threats.