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