This chapter provides a brief overview of constructivist international relations theory and explores how it can help explain two of the most important transformations in international security over the last century: the growing belief that the “human” should be an object of security, and the expanding metropolis of different kinds of actors whose goal is to produce security for all individuals in the name of “humanity” and the “international community.” The introduction to the chapter provides a quick background to the sociological context that gave birth to constructivism. Section 7.2 provides a brief conceptual overview of constructivism, with particular attention paid to those attributes that might be useful for students of international security and that will be relevant for the historical analysis in Section 7.3, which concerns the expansion of international security from a state-centric exercise to a growing role for the “international community” to defend “humanity.”