The purpose of this design experiment was to research, test, and iteratively derive principles of word learning and word organization that could help to theoretically advance our understanding of vocabulary development for low-income preschoolers. Six Head Start teachers in morning and afternoon programs and their children (N = 89) were selected to participate in the World of Words, a 12-min daily supplemental vocabulary intervention; six classes (N = 89) served as a comparison group. Our questions addressed whether the difficulty of words influenced the acquisition and retention of words and whether learning words in taxonomies might support vocabulary development and inference generation. We addressed these questions in two design phases for a total intervention period of 16 weeks. Pre- and post-unit assessments measured children's expressive language gains, categorical development, and inference generation. Significant differences were recorded between treatment and comparison groups on word knowledge and category development. Furthermore, children in the treatment group demonstrated the ability to infer beyond what was specifically taught. These results suggest that instructional design features may work to accelerate word learning for low-income children. (Contains 2 notes, 6 tables, and 2 figures.)