Imagine it is the year 2100 on planet Earth. Human life has dramatically changed, particularly with a view to the irreversible effects of climate change. Most of the catastrophic effects on the environment in 2100 originated in the first decades of the twenty-first century. The book starts its introductory chapter with a peak into the future of the year 2100 and a thought experiment of a time traveller from this year who comes to confront the representatives of the UN member States in the year 2025 with their intergenerational obligations. The time traveller asks them: “Why do you act in explicit violation of your responsibilities under the concept of intergenerational equity?” The chapter then delimits the scope of the book’s analysis by describing what is understood by intergenerational equity in a context of environmental issues. Besides the long-term effects of anthropogenic climate change, many other environmental problems risk threatening the needs of future generations—ranging from areas as diverse as nuclear waste, conservation of renewable resources and biodiversity loss. Starting from the current state of the art in jurisprudence and literature, the introductory chapter describes the need for further research on intergenerational equity. It introduces the two-fold research question addressed by this book: firstly, on the status quo of intergenerational equity and, secondly, taking an intertemporal perspective. The book attempts to answer which legal regime of intergenerational equity is temporally applicable to address the open issues of the intertemporal legal relationship between present and future generations. After explaining the methodological approaches of the book, the introductory chapter eventually gives an outline of how the work will be carried out and structured.