The effects of recent moves toward national testing regimes are being felt at the classroom level, where teachers feel compelled to "teach to the tests." Thus, they are now accountable in two ways: to students (and their understandings) and to the public and to the school boards (for improving overall student test scores). It is important to understand how teachers assess their students in response to these pressures. In this article, the authors report on findings from a year-long study of how teachers determine student understanding and why they employ the instructional and assessment strategies that they do. Qualitative data were collected on five science teachers from the Sonora Foothills School District, and are reported here in the form of individual case studies. The authors single out three important issues raised by their research: (1) the importance of recognizing the impact of political pressures on classroom teachers; (2) the importance teachers place on connecting teaching and assessment; and (3) the accountability dilemma and teachers' capacity to operate within a complex policy milieu.