This book examines same-sex unions policy (SSU) developments in eighteen western democracies and seeks to explain why the overwhelming majority of these countries has implemented a national law to recognise gay and lesbian couples since 1989. Drawing on extensive interview and document analysis the book illustrates the ways in which SSU policy debates and outcomes have been catalysed by international norm diffusion and social learning. The second part of the study analyses these processes in greater depth using two comparative case studies (Germany and the Netherlands; the United States and Canada) to identify how the norm influences domestic policy debates as well as which factors determine how much power it can exert in different national environments. The case study analysis also reveals why western democracies have implemented different models of recognition (marriage vs. registered partnership vs. unregistered cohabitant).