Pre-service teachers face many challenges as they learn to teach in ways that are different from their own educational experiences. Pre-service teachers often enter teacher education courses with pre-conceptions about teaching and learning that may or may not be consistent with contemporary learning theory. To build on preservice teachers' prior knowledge, we need to identify the types of views they have when entering teacher education courses and the views they develop throughout these courses. The study reported here focuses specifically on preservice teachers' views of their own students' prior knowledge and the implications these views have on their understanding of the formative assessment process. Sixty-one preservice teachers were studied from three sections of a science methods course. Results indicate that preservice teachers exhibited a limited number of views about students' prior knowledge. These views tended to privilege either academic or experience-based concepts for different aspects of formative assessment, in contrast to contemporary perspectives on teaching for understanding. Rather than considering these views as misconceptions, it is argued that it is more useful to consider them as resources for further development of a more flexible concept of formative assessment. Four common views are discussed in detail and applied to science teacher education. (Contains 6 figures and 3 tables.)