This chapter examines the shift of security concepts away from the traditional focus on war and the State in a Westphalian international system, discussing the evolution of current security concepts in their historical and theoretical context. It shows how State-based security concepts have slowly made way for broader understandings of security over the past decades. Security is no longer seen exclusively as a matter of the State; violent conflicts are more often than not intra-State or internationalized in nature and collective security arrangements have not always been effective in crisis situations. The chapter then argues that the successive broadening and deepening of security concepts has led to issues of coherence (and lack thereof) and concept stretching in the research field, while leaving crucial dynamics of contemporary security governance underexplored. It highlights several challenges posed by the ongoing and rapid transformation of global security dynamics and discusses their implications for international law.