The purpose of this study was to examine the hypothesis that helping preschoolers learn words through categorization may enhance their ability to retain words and their conceptual properties, acting as a bootstrap for self-learning. We examined this hypothesis by investigating the effects of the World of Words instructional program, a supplemental intervention for children in preschool designed to teach word knowledge and conceptual development through taxonomic categorization and embedded multimedia. Participants in the study included 3- and 4-year-old children from 28 Head Start classrooms in 12 schools, randomly assigned to treatment and control groups. Children were assessed on word knowledge, expressive language, conceptual knowledge, and categories and properties of concepts in a yearlong intervention. Results indicated that children receiving the WOW treatment consistently outperformed their control counterparts; further, treatment children were able to use categories to identify the meaning of novel words. Gains in word and categorical knowledge were sustained six months later for those children who remained in Head Start. These results suggest that a program targeted to learning words within taxonomic categories may act as a bootstrap for self-learning and inference generation. (Contains 2 notes, 10 tables, and 1 figure.)