Philosophers, writers, self-help gurus, and now scientists have undertaken the challenge of how to foster greater happiness. In this chapter, we first discuss the different ways that happy and unhappy individuals construe their worlds, respond to social comparisons, make decisions, and self-reflect. Next, we examine whether deliberate strategies to improve happiness can be effective, and consider factors that may curtail their effectiveness. Specifically, we review evidence from randomized controlled experiments indicating that people can increase their happiness by practicing simple positive activities with effort and commitment. Such activities—including performing kind acts, expressing gratitude or optimism, and re-experiencing joyful events—represent the most promising route to enhanced happiness. We also discuss the optimal conditions under which positive activities increase happiness, and the mechanisms that underlie their success. Future researchers must continue not only to investigate which particular practices make people happier, but how and why they do so.