Conceptual Change is a widely accepted theoretical framework for science education. Setting up successful learning and teaching arrangements in this framework necessarily entails including students´ preconceptions into the construction of those arrangements. In order to provide a basis for such arrangements this study investigated and explored preconceptions about radiation in a qualitative case study with seven students (age 17-18). The use of grounded theory allowed the discovery of deep-rooted conceptions and the formulation of a theory to help learners understand the basic concepts of radiation more quickly and more reliably. The results revealed a new conception "the nature of radiation", which hampers students' understanding and interferes with their explanations of radiation. Additionally, analysis of the data revealed interesting conceptions concerning the danger of different types of radiation. These highly interesting findings about the nature of radiation will be embedded in a broader perspective and suggestions for learning and teaching about radiation will be given.