This study examined the effects of social class background on children's oral language development. The study focused on children's oral vocabulary knowledge because such knowledge provides a good summary of language skills, is easily measured, and is very consequential for later schooling success. Using a large national data set, the study examined: (1) What is the pattern of growth of oral vocabulary knowledge? In particular, how do vocabulary growth rates differ in preschoolers and school-age children?; (2) What is the timing and magnitude of the effect of parental social class on children's vocabulary growth? At what age do social class differences in vocabulary knowledge first appear? How large are they? How does this social class effect evolve as children age? In particular, does it diminish when children move outside the home and begin attending school?; and (3) To what extent can social class differences in children's vocabulary knowledge be explained by the linguistic cultural capital of their parents--the explicit skills and habits that parents employ in their child-rearing behaviors? And how do these parental cultural capital effects vary as the child moves through preschool and school? The study found that significant social class differences in oral vocabulary growth emerge at the very earliest ages and attain a substantial magnitude by 36 months of age. These social class differences continue to widen at ages 3 and 4, although this occurs more strongly among African-Americans than Whites. Approximately half of these social class differences in vocabulary growth rates can be attributed to the differential family linguistic instruction provided by mothers of varying social classes. By age 5 and above, vocabulary growth rates are relatively similar across social classes, suggesting that attendance in kindergarten and later grades has an equalizing effect as children from lower social strata are exposed to teacher and peer interaction and school instruction. (EV)