This chapter examines arguments over government paternalism derived from considerations of individual well-being. It begins with a discussion of the classical economic model of rationality and the emergence of an alternative model during the twentieth century. It then reviews an increasing volume of evidence from behavioral economics and psychology of the so-called “reasoning failure”: the fact that individuals, in trying to achieve the end of improving their well-being, tend to make mistakes and do so in a systematic way. Four possible sources for such failure are discussed: limited technical ability, limited imagination, limited willpower, and limited objectivity. The chapter concludes by rejecting ends-related paternalism but accepting the well-being case for means-related paternalism.