This study examines the organization and development of 5 domains of reasoning (categorical, quantitative, spatial, causal, and propositional) and the construct validity of a test designed to measure development from early adolescence to early adulthood. The theory underlying the test is first summarized and the conceptual design of the test is then illustrated. Each domain was addressed by tasks tapping abilities known to be acquired in this age period. The test was administered to 629 adolescents ranging in age from 12 to 18 years. Confirmatory factor analysis validated the 5 domains of reasoning and revealed a common factor underlying all domains. The Rasch model was used to scale the items and specify the reliability of the test across the whole sample and within different groups of participants (female, male, students of gymnasium, and students of lyceum). This model showed that the test is highly reliable and invariant across groups. Cluster analysis and the saltus model were applied to uncover successive developmental stage-like levels of difficulty and showed the presence of five such levels. The procedural and representational characteristics of these levels were also specified and their implications for developmental and cognitive theory were discussed.