The relationship between word knowledge and reading comprehension has been well documented in research; however, the nature of this relationship remains unclear. Researchers usually distinguish between 2 aspects of an individual's word knowledge: breadth and depth. In addition to these 2 factors, it may be important to also consider fluency in the study of word knowledge. Two hundred and three 3rd-grade students took part in a study that examined the relationships between 3 dimensions of word knowledge and reading comprehension. Confirmatory factor analyses, structural equation modeling, and hierarchical regression analyses show that a 2-factor model of breadth and depth/fluency provides the best fit to the data. Breadth has a stronger relationship to reading comprehension than does depth/fluency; however, the 2 dimensions of word knowledge have significant overlapping variance that contributes to the prediction of reading comprehension.