The primary goal was to expand our understanding of text-reading fluency (efficiency or automaticity): how its relation to other constructs (e.g., word reading fluency, reading comprehension) changes over time and how it is different from word-reading fluency and reading comprehension. The study examined (a) developmentally changing relations among word-reading fluency, listening comprehension, text-reading fluency, and reading comprehension; (b) the relation of reading comprehension to text-reading fluency; (c) unique emergent literacy predictors (i.e., phonological awareness, orthographic awareness, morphological awareness, letter name knowledge, vocabulary) of text-reading fluency versus word-reading fluency; and (d) unique language and cognitive predictors (e.g., vocabulary, grammatical knowledge, theory of mind) of text-reading fluency versus reading comprehension. These questions were addressed using longitudinal data (two time points; mean age = 5 years 2 months and 6 years 1 month, respectively) from young Korean speaking children (N = 143). Results showed that listening comprehension was related to text-reading fluency at time 2 but not at time 1. At both times, text-reading fluency was related to reading comprehension, and reading comprehension was related to text-reading fluency over and above word-reading fluency and listening comprehension. Orthographic awareness was related to text-reading fluency over and above other emergent literacy skills and word reading fluency. Vocabulary and grammatical knowledge were independently related to text-reading fluency and reading comprehension, whereas theory of mind was related to reading comprehension but not text-reading fluency. These results reveal the developmental nature of relations and mechanisms of text-reading fluency in reading development. An appendix presents a plot showing the distribution of rapid automatized naming task. [This paper was published in "Reading Research Quarterly" (EJ1075040).]