In this book the development of an instrument for the testing of students' procedural knowledge on scientific inquiry is described. The test instrument, called NAW-L test, was designed to measure differences in performance between German grade 5 students from the school type "Hauptschule" on the one hand and from thy type "Gymnasium" on the other. It was developed on the basis of a competence model, which predicts task difficulty. The quality of the NAW-L test was determined by Rasch analysis. Typical deficits of younger students in scientific reasoning and scientific inquiry mentioned in the literature could be replicated in this project, speaking for the validity of the test instrument.