In this paper, we examine the NCLB definition of a "highly qualified" teacher, the combined nature of federal and state highly qualified mandates, and how science teachers and their principals in one large, urban district have responded. In particular, we report on the kinds of licenses that teachers of science hold; the relationship between science teachers' qualifications and teaching assignments, both within and outside of their licensure; and the extent to which these trends vary across different student populations, subject disciplines, and over time. In addition, we present principal- and teacher-interview data from eight case-study schools and district-wide teacher-survey data to shed light on our findings and show how the NCLB definition of highly qualified is understood and regarded by teachers themselves. The data show that progress has been made in assigning teachers with demonstrated content knowledge in the specific science subjects they are assigned to teach, however the rate of change is slower for some students populations and science disciplines. Moreover, both teachers and principals adapt to these mandates in a variety of ways, some of which reflect the intended and unintended consequences of these policies. (Contains 1 table, 2 figures, and 7 notes.)