Existential risks are those that threaten the extinction of humanity, or, somewhat more broadly, those "where an adverse outcome would either annihilate Earth-originating intelligent life or permanently and drastically curtail its potential" (Bostrom, 2002). While it has long been widely recognized that global nuclear war and catastrophic climate change have potentially civilization-threatening consequences, it is only in the last decade or two that scholars have begun, in work that is typically highly interdisciplinary, to systematically investigate a broader range of existential risks. The papers in this ebook discuss these issues.