It is well established that children's reading comprehension is driven, at least in part, by their awareness of morphemes, or the smallest units of meaning in language. The question of how it does so is largely open; this mechanistic knowledge would specify theories of reading comprehension and guide effective classroom instruction. We report here on a longitudinal study designed to test two candidate mechanisms by which morphological awareness might support the development of reading comprehension: use of morphemes to read words and to understand words, known as morphological decoding and analysis, respectively. We tracked the development of 221 children on key measures from Grades 3 to 5. These three-wave longitudinal data enabled us to test morphological decoding and analysis as mediators connecting morphological awareness to gains in reading comprehension over time. Structural equation modeling showed that morphological awareness contributed to gains in both morphological decoding and analysis. Critically, only morphological analysis mediated the contribution of morphological awareness to gains in reading comprehension between Grades 3 and 5. These findings elaborate predictions of the morphological pathways framework. Specifically, longitudinal modeling shows how morphological awareness supports children's developing reading comprehension: by enabling the use of morphemes to read and understand words, with effects on the understanding of words supporting children's growing skill in understanding texts. Evidence supporting these two specific mechanisms inspires the design of targeted teaching on morphemes toward the development of strong reading comprehension.