This chapter examines how happiness is measured and why different tools can yield divergent conclusions. It traces the rise of self-reports in the “Happiness Revolution,” where subjective well-being indicators joined, and sometimes challenged, GDP and longevity as core evidence. The chapter then reviews major instruments, from single global questions and the Cantril Ladder to multi-item batteries such as the Satisfaction with Life Scale, PANAS, the Subjective Happiness Scale, Oxford measures, and change-sensitive indices like the Steen Happiness Index and Authentic Happiness Inventory. It highlights how recall windows, item wording, response formats, and cultural adaptation shape what respondents actually report and what inferences are warranted. Finally, it summarizes key limitations of self-reports—constructed judgments, shifting standards, and response-style differences—and presents practical guidance on triangulating survey data with experience sampling, informant reports, behavioral and physiological indicators, and qualitative methods so that happiness measures can be selected, interpreted, and reported in a transparent, decision-relevant way.