Software companies and startups often follow the idea of flourishing happiness among developers. Perks, playground rooms, free breakfast, remote office options, sports facilities near the companies, company retreats, you name it. The rationale is that happy developers should be more productive and also retained. But is it the case that happy software engineers are more productive? Moreover, are perks the way to go to make developers happy? Are developers happy at all? What are the consequences of unhappiness among software engineers? These questions are important to ask both from the perspective of productivity and from the perspective of sustainable software development and well-being in the workplace. Managers, team leaders, as well as team members should be interested in these concerns. This chapter provides an overview of our studies on the happiness of software developers. You will learn why it is important to make software developers happy, how happy they really are, what makes them unhappy, and what is expected regarding happiness and productivity while developing software.