In Experiments 1 and 2 first-, third-, and seventh-grade children and college subjects circled the letter a while reading passages constructed of words familiar to first graders. First graders made more errors on the letter a embedded in a word than on the word a, whereas the converse was true of the other age groups. In Experiments 3 and 4 first-, second-, fourth-, and seventh-grade children and college students read passages and circled the letter t, making more errors on the common word the than on other words and on correctly spelled than on misspelled words. The effect of misspelling the other words increased with age and reading skill. Our combined results suggest that reading unit size increases with age and reading ability and that, whereas younger children, like adults, unitize common words, the unitization of less common words increases as word configurations become more familiar.