Background/Context: Although there is substantial evidence that high-quality professional development can improve teacher practices, less evidence exists for the effects of teacher professional development on intermediate outcomes, such as teacher practices, and their ultimate effects on K-12 student achievement. This work links professional development through teacher practices to examine their separate and combined effects on student achievement. Research Questions: When teacher characteristics and teacher preparation program are controlled, what are the effects of teacher professional development on (1) teacher practices in mathematics and reading, and (2) subsequent student mathematics and reading achievement? Population: The study uses students (n = 1,550-6,408) nested within teachers (n = 168-1,029) from six existing databases, two from the 2000 Beginning Teacher Preparation Survey conducted in Connecticut and Tennessee, and four from the National Assessment of Educational Progress (NAEP Mathematics 1996, 2000, and NAEP Reading 1998, 2000). Research Design: This quantitative study employed a hybrid structural equation model built based on relationships indicated by the literature. Using extant large-scale data sets, the model was first tested using the smallest data set and then confirmed using successively larger state and national data sets. Conclusions: Professional development has moderate effects on teacher practice and very small but sometimes significant effects on student achievement when the effects of professional development are mediated by teacher practice. In spite of differences in samples, academic subjects, and assessments, the effects of professional development on teacher practice and student achievement persist and are remarkably similar across analyses.