The process of thinking in and about models as a scientific practice should be integrated into science teaching and learning. Empirical studies show that students see models primarily in their role as media to facilitate content learning while rarely appreciating models as instruments of scientists which allow the deduction and the testing of predictions. In order to foster students' meta-modeling knowledge successfully, specific diagnostic information about students' understanding of models and modeling is needed. The aim of this study is to gather such diagnostic information by investigating students' understanding of the nature and the purpose of models in biology with respect to context- and grade-specific differences. Students' understanding of the nature and the purpose of models was assessed by using forced choice tasks (N[subscript students] = 285). In order to gain qualitative insight into possible reasons for context-specific differences, the students were additionally asked to give reason for their decision in the forced choice tasks by answering open-ended justification tasks. The results indicate that the majority of students in all grades see models as idealized representations of an original which have the purpose to show or to describe this original. Students' levels of understanding of the nature and the purpose of models increase only little across grades. Nevertheless, a grade-specific analysis of the consistency of students' understanding across contexts suggests that the students' understanding becomes more consistent in higher grades. Students' justifications helped to identify model contexts which have a high potential to be fruitful when trying to foster students' understanding of the nature and the purpose of models.