One of the most striking findings from happiness research is that most of us are at least mildly happy most of the time. There is a strong argument that this chronic level of moderate happiness is evolutionarily adaptive. This chapter explores why positive emotions and happiness might be adaptive. It considers Fredrickson’s (1998, 2001) Broaden and Build Theory (BBT), which hypothesizes that positive emotions (happiness) help us build useful skills and live better and happier lives. Positive emotions make powerful contributions to our overall well-being. Feelings of joy, contentment, love, or interest, for example, are not just signs that we are flourishing. They are also causes of flourishing. The chapter examines this and other features of the positive emotion-well-being relationship. The appropriate ratio of positive to negative emotions can lead to an upward spiral of positivity that increases well-being.