In this article we describe how an underperforming school district used research and theory on curriculum, assessment, implementation, and school and classroom organization to develop and implement district standards and improve the achievement of elementary school students. Key reforms included teachers developing essential curriculum standards, standards-based criterion-referenced tests, and standards-based extended learning opportunities. Teachers rated the reform efforts positively and reported a high likelihood of implementation. Using California Department of Education data, we employed econometric analyses to estimate program effects for the district's elementary schools from 1999 to 2002. A difference-of-differences model estimated 10 of 16 positive and significant effects on grades 2 to 5 SAT9 scaled score mathematics achievement, ranging from .2[sigma] to .7[sigma]. The same model applied to reading scores showed 12 of 16 insignificant or negative grade-level effects. As predicted, however, second and third graders in 2002 -- those students who had 3 and 4 years of exposure to the program exclusively -- experienced only positive (4 of 4) and mostly significant (3 of 4) reading and mathematics effects. Results may guide district administrators implementing standards or comprehensive school reforms.