The goal of this research was to gain a more comprehensive understanding of how cognitive flexibility (CF) develops in early childhood. Previous research on cognitive flexibility development has tended to focus solely on studying 3- to 4-year-olds on a single paradigm that involves children switching from one task to another while resolving response conflict. For example, children switch from sorting coloured shapes by a rule (e.g., colour) to sorting the same stimuli by a new rule (e.g., shape). This has led to the pervasive, but simplistic idea that children achieve CF when they overcome perseveration at age 4 (e.g., Munakata, 2012; Zelazo et al., 2003). Consequently, theoretical accounts of CF development have focused on explaining why 3-year-olds perseverate. Perseveration has been explained as either a failure of working memory or a failure of inhibitory control, and little progress has been made in testing between these two accounts. Three approaches were combined in the current research to address this issue: 1) A new paradigm was used to study cognitive flexibility, capable of examining different types of flexible behaviour; 2) 2-year-olds were studied for the first time on measures of CF to learn about its emergence; and 3) both individual differences studies and randomised control training studies were used to examine how working memory and inhibitory control contribute to the development of CF. Together, the findings challenge the prevailing view that CF development begins with perseveration at age 3 and flexible behaviour at age 4 due to increases in working memory or inhibitory control by showing that: 1) overcoming response conflict is not the only way children can demonstrate flexible behaviour; 2) key developments in CF occur prior to age 3; and 3) both working memory and inhibitory control contribute to CF development depending on the task demands. Collectively, the findings provide a more comprehensive and nuanced account of cognitive flexibility development.