Most research on children’s syntactic development has focused on commonalities among children rather than individual differences between children. Yet, recent studies have shown differences in syntactic development between children from higher and lower socioeconomic status (SES) backgrounds. These studies have demonstrated that SES differences in the complexity of input at home and school contribute to the variability in syntactic development. In this talk, I discuss three potential mechanisms that may help account for why SES differences in input affect syntax: a child’s vocabulary, language processing, or a combination of both. First, SES may affect syntactic development through vocabulary development: differences in lower level lexical representations have cascading effects on higher-level syntactic representations. A second distinct, but not mutually exclusive possibility is that SES affects syntax through children’s ability to use their input, or their language processing skills. By measuring children’s syntactic revision skills using a passive sentence processing task, I argue that SES differences in this skill also influence syntax. The third possibility is that a combination of vocabulary and processing skills best explain SES differences in syntax. To test these hypotheses, I present data from a study that measured syntax, vocabulary, and processing among a sample of five-year-old children from diverse SES backgrounds. From these data, I present evidence that, on average, children from higher SES backgrounds have 1) larger vocabularies and, 2) better syntactic revision skills, and argue that, together, these two mechanisms account for SES differences in syntactic skills.