Code-switching is a common feature of bilingual language use and has multiple factors that influence the frequency and type of code-switching. 56 Spanish-English bilingual children recorded sessions of Spanish-designated and English-designated interactions with a caregiver at 2.5 and 3.5 years. These sessions were transcribed and coded for all code-switched utterances. At both ages, we found: (1) Children switched to English more frequently than they switched to Spanish. (2) Their degree of English dominance was a positive predictor of their frequency of switching to English, but anegative predictor of their frequency of switching to Spanish. Between 2.5 and 3.5 years, children became more English dominant, and their rate of switching to English increased while their rate of switching to Spanish decreased. The present findings suggest that the strongest influence on bilingual children’s code-switching is their relative proficiency in their two languages and as that proficiency changes, their code-switching changes.