The Authentic Happiness Inventory (AHI) is a frequently used measure for the subjective assessment of happiness and is primarily used in positive psychology intervention studies. It has been argued that it is sensitive to detect subtle changes in happiness and differentiates happiness at very high levels. We designed a series of studies to test some of the basic premises and to assess the reliability and validity of the German version of the AHI (total N = 5166). In Study 1, four independently collected samples provide evidence for its good psychometric properties and convergent as well as discriminant validity. Study 2 shows that the AHI has high test-retest correlations over a period of one week, and one, three and six months (r = .75–.85; N = 319). Also, the experience of positive life events went along with higher scores in the AHI. In Study 3, the AHI was used in a positive psychology intervention study by testing two well-established positive psychology interventions (i.e., “another door opens”, and “three good things”) against a placebo control group (N = 400 in total). Results show that the AHI reflects the expected changes in well-being (i.e., increase in the intervention in comparison with the placebo-control group). Overall, the three studies support the notion that the AHI has good psychometric properties and provides support for its validity. Potential further applications of the measure are discussed.