This dissertation addresses various aspects of the question "What does it mean to be human?", approaching the issue from biological, philosophical and ethical perspectives. The discussion focuses primarily on the philosophical and ethical implications of the transhumanist movement, an intellectual community seeking to favorably transform the human organism via the safe deployment of interventions such as genetic engineering and pharmacologic enhancement. Following a review of bioethical principles, the dissertation begins by examining the notion of "personhood" in philosophical, historical and biological contexts. Next, the possibility that developments in neuropharmacology might ultimately lead to an artificial paradise free of the negative aspects of the human condition (but without the often destructive effects of today's mood altering drugs) is considered. A philosophical difficulty related to the existentialist notion of "authenticity" in such a "mood optimized" synthetic existence is identified. © © U Un ni iv ve er rs si it ty y o of f P Pr re et to or ri ia a iv This discussion is followed by a rather technical exposition on the occasional difficulties of establishing when a person is alive or dead, particularly in the setting of "brain death". A number of philosophical flaws with the notion of brain death as it is used currently are presented. The discussion then considers some of the philosophical issues raised by the possibility of human cryonic suspension. A distinction is made between the information preserved in a person's brain and the substrate used to hold that information. Implications for personhood are also discussed, as well as a number of related ethical issues. Finally, objections raised by "bioconservative" critics of transhumanism are critically examined and found to be for the most part unconvincing, frequently relying on emotion and intuition rather than on evidence, logic and reason.