This book provides a comparative, neo-institutionalist approach to the different factors impacting state adoption of?or refusal to adopt?same-sex marriage laws. The now twenty-one countries where lesbians and gay men can legally marry include recent or longstanding democracies, republics and parliamentary monarchies, and unitary and federal states. They all reflect different positions with respect to religion and the cultural foundations of the nation. Countries opposed to such legalization, and those having taken measures in recent years to legally reinforce the heterosexual fundaments of marriage, present a similar diversity. This diversity, in a globalized context where the idea of same-sex marriage has become integral to claims for LGBTI equality and indeed LGBTI human rights, gives rise to the following question: which factors contribute to institutionalizing same-sex marriage?0The analytical framework used for exploring these factors in this book is neo-institutionalism.