In this article, we present an analysis of relations between (a) student ac
hievement in the domain of fractions and (b) the extent to which classroom
practices are aligned with principles recommended by current reform framewo
rks (e.g., National Council of Teachers of Mathematics, 1989). Hierarchical
linear model analyses were performed on classroom observation and pre- and
postinstruction achievement data collected in 19 upper elementary classroo
ms. These analyses revealed that alignment of classroom practices with refo
rm principles was related to student achievement in problem solving but not
in computation; furthermore, the relation differed for students who began
instruction with different levels of prior knowledge as indexed by a pretes
t measure. For students who began with a rudimentary understanding of fract
ions, the relation between measures of classroom practice and problem solvi
ng was linear. In contrast, for students who began without a rudimentary un
derstanding of fractions, the relation was nonlinear; in classrooms rated l
ow on alignment with reform principles, performance on problem-solving item
s was near floor but increased at a certain threshold level of alignment. T
he findings demonstrate the value of reform principles as a guide for effec
tive practice as well as the importance of a coordinated analysis of studen
ts' prior understandings and classroom practices in investigations of child
ren's learning.