Code-switching refers to the alternating use of two or more languages, either within a sentence (intrasentential) or between sentences (intersentential). Because code-switching is governed by grammatical rules, both language systems are presumed to be active while producing mixed sentences. Code-switching occurs in discourse of fluent and nonfluent bilinguals alike, although intrasentential switches are often thought to be illustrative of the level of bilingualism or comfort with another language (e.g., Poplack, 1980). This chapter focuses on code-switching in preschool children, with a special emphasis on the linguistic context, comparisons between intra- and intersentential switching, the effect of level of bilingualism, differences in production and comprehension of sentences, and experimental demands. To this end, we present new data from 36 Spanish-English bilingual children between the ages of 3 and 5, and tie the results to the theoretical framework of Kroll and her colleagues (e.g., Kroll & Stewart, 1994; Sholl, Sankaranarayanan, & Kroll, 1995) on conceptual and lexical links between words in sentence processing.