Text reading fluency refers to the ability to read connected texts with accuracy, speed, and expression (prosody), and has garnered substantial attention as an important skill for reading comprehension. However, two fundamental questions remain--the dimensionality of text reading fluency including text reading efficiency (accuracy and speed) and reading prosody, and the directionality of the relation between text reading fluency and reading comprehension. These questions were addressed using longitudinal data from Grade 1 (M[subscript age] = 6.36 years) to Grade 3 (M[subscript age] = 8.34 years). Majority of children were White (approximately 60%) and African American (26%) with 39% to 52% from low-SES backgrounds, depending on the grade. Text reading fluency, word reading, listening comprehension, and reading comprehension were measured. Results from confirmatory factor analysis revealed that text reading fluency is a multidimensional construct with a trifactor structure, which has a general factor that captures common ability across text reading efficiency and reading prosody as well as local and specific factors that are unique beyond the general factor. However, the general factor was the most reliable factor, whereas local and specific factors were not reliable. The directionality of the relation between text reading fluency and reading comprehension was addressed by examining two competing structural equation models--text-reading-fluency-as-a-predictor/mediator model and text-reading-fluency-as-an-outcome model--and data supported the former. These results indicate that text reading fluency is a multidimensional construct, and it acts as a predictor, mediating the relations of word reading and listening comprehension to reading comprehension. [This article was published in "Developmental Psychology" (EJ1300306).]