Proposals to make us smarter than the greatest geniuses or to add thousands of years to our life spans seem fit only for the spam folder or trash can. And yet this is what contemporary advocates of radical enhancement offer in all seriousness. They present a variety of technologies and therapies that will expand our capacities far beyond what is currently possible for human beings. In Humanity's End, Nicholas Agar argues against radical enhancement, describing its destructive consequences. Agar examines the proposals of four prominent radical enhancers: Ray Kurzweil, who argues that technology will enable our escape from human biology; Aubrey de Grey, who calls for anti-aging therapies that will achieve "longevity escape velocity"; Nick Bostrom, who defends the morality and rationality of enhancement; and James Hughes, who envisions a harmonious democracy of the enhanced and the unenhanced. Agar argues that the outcomes of radical enhancement could be darker than the rosy futures described by these thinkers. The most dramatic means of enhancing our cognitive powers could in fact kill us; the radical extension of our life span could eliminate experiences of great value from our lives; and a situation in which some humans are radically enhanced and others are not could lead to tyranny of posthumans over humans.