Measurement of competencies in science is an important tool to inform school administrators and educators about the status of the school system and a realistic view on mean student abilities. As consequences there might be changes in the curriculum or the standardization of the national education standards. The content used in the items for the assessment defines the subject which is measured and influences the item difficulty. Therefore the role of content for the assessment needs to be discussed. In this chapter authors from two large assessment projects in Germany and Switzerland present the respective assessment and how content validity is ensured and the content’s influence on difficulty is investigated. The first two authors briefly introduce to the assessments’ frameworks. The following two authors report about research studies conducted within the second assessment project.