Children's vocabulary development represents a critical foundation for their reading skills. This article describes a curriculum-based vocabulary progress monitoring tool that was used to track the development of Head Start children--and particularly those at highest risk for later reading difficulty--as they learned new words during the school year and then presents evidence that the progress monitoring tool measures the same underlying construct as a standardized assessment of vocabulary. Further, we explain how teachers used these data to adjust their instruction for individual children and then describe children's growth on the measure. Finally, we provide practical guidance about how this vocabulary progress monitoring tool could be strengthened, implemented, and further evaluated in other contexts. (Contains 2 tables and 1 figure.)