Abundant research has pointed to the importance of non-cognitive skills for success in life. This paper describes the development and validation of the “UiL”, designed to measure 19 non-cognitive skills that have been identified as being important for school children in Denmark. First, we describe the development of the scales, and then report a preliminary validation with 1560 students from Grades 4–9. Second, we present a validation of the final UiL, which was revised and re-administered to the same sample of students. The final validation sample consisted of 1373 students (48.6% boys, ages 9–16 years). Results from the Confirmatory Factor Analysis indicated that the UiL had acceptable discriminant and convergent validity. The results from a RASCH partial credit analysis indicated that four of the 19 had excellent fit, with other scales needing nuanced interpretation because of some item misfit, local dependence, multidimensionality, and DIF by grade.