The present study explored whether different executive control and speed measures (working memory, inhibition, processing speed, and naming speed) independently predict individual differences in word reading and reading comprehension. Although previous studies suggest these cognitive constructs are important for reading, the authors analyze the constructs simultaneously to test whether each is a unique predictor. Latent variables from 483 participants (ages 8-16 years) were used to portion each cognitive and reading construct into its unique and shared variance. In these models 2 specific issues are addressed: (a) Given that the wide age range may span the theoretical transition from "learning to read" to "reading to learn," the authors first test whether the relation between word reading and reading comprehension is stable across 2 age groups (ages 8-10 and 11-16); and (b) the main theoretical question of interest: whether what is shared and what is separable for word reading and reading comprehension are associated with individual differences in working memory, inhibition, and measures of processing and naming speed. The results indicated that (a) the relation between word reading and reading comprehension is largely invariant across the age groups, and (b) working memory and general processing speed, but not inhibition or the speeded naming of nonalphanumeric stimuli, are unique predictors of both word reading and comprehension, with working memory equally important for both reading abilities and processing speed more important for word reading. These results have implications for understanding why reading comprehension and word reading are highly correlated yet separable. (Contains 5 tables, 6 figures and 5 footnotes.)