Developmental differences in object word learning and use were investigated. Two groups of infant speakers: Beginner (vocabularies under 50 words) and Advanced (vocabularies over 50 words), participated in a concept formation and word learning and generalization experiment. Results indicated that both speaker groups formed concepts of and learned words for the to-be-learned objects, and generalized these words. However, Advanced speakers learned more concepts and words, and engaged in broader generalization than Beginner speakers. The following variables did not account for these developmental differences: child age, object permanence development, maternal speech, child action on objects. These findings can be explained by developmental differences in both the number and kind of object concepts formed in the second year. Words were primarily acquired for object concepts by both speaker groups. However, Beginner speakers formed fewer concepts, thus having a less extensive conceptual base supporting word learning. This may account for their lower levels of vocabulary acquisition. Additionally, Beginner speakers appear to form prototype/exemplar-based object concepts, leading to limited word extension, while Advanced speakers form featurally based object concepts, leading to broader extension.