We examined the role of a hypothesized factor in reading comprehension: morphological awareness, or the awareness of and ability to manipulate the smallest meaningful units or morphemes. In this longitudinal study, we measured English-speaking children's morphological awareness, word reading skills, and reading comprehension at Grades 3 and 4, in addition to their phonological awareness, vocabulary, and nonverbal ability as control measures. Path analyses revealed that word reading skills partially mediated the relationship between morphological awareness and reading comprehension at each grade. Further, children's early morphological awareness partially explained children's gains in reading comprehension, and their early reading comprehension partially explained their gains in morphological awareness. These findings support the predictions of recent models of reading comprehension: that morphological awareness impacts reading comprehension both indirectly through word reading skills and directly through the language system and that morphological awareness underpins the development of reading comprehension (e.g., Perfetti, Landi, & Oakhill, 2005).