Recent research has demonstrated an advantage for bilingual preschoolers on a number of tasks related to the control of behavior and attention (executive functions). However the exact nature of an eventual advantage is not known. The present study compared the performance of preschool children who were regularly exposed to two or more languages, with children who were only exposed to one language, on tasks that place demands on vocabulary, reasoning, response inhibition, flexible rule use, and visual working memory span.The objective of the investigation was to explore the question of how experience with more than one language may be related to the development of cognitive flexibility, or the ability to generate and shift ideas and responses. Our hypothesis was that the bilingual children would have an advantage related to cognitive flexibility as measured on the DCCS-AG, a task related to flexible rule use in the presence of distraction. We were interested in differences between the groups related to response inhibition and working memory, for the purpose of identifying the component of the problem responsible for an eventual bilingual advantage. We were additionally interested in the interrelatedness of the tasks, for support this might provide for alternative accounts of the development of executive functions.Our hypothesis was not supported: the bilingual children did not have an advantage with regard to cognitive flexibility. Among the youngest children, the bilinguals had a significant advantage in visual working memory span, but this advantage disappeared as the children got older, and did not reach significance in the overall sample. Among children older than 4 ½ years, monolingual children had a significant advantage in tasks related to flexible rule use and vocabulary. Tasks related to response inhibition and vocabulary correlated with the flexibility task. When we had controlled for the effects of age, only vocabulary correlated with the flexibility task. The interrelatedness of the tests is discussed in terms of various accounts of the structure of executive functions in development. The role of language in the development of cognitive flexibility is considered.