Cognitive flexibility has been described as the mental ability to switch between thinking about two different concepts, and to think about multiple concepts simultaneously. Despite some disagreement in the literature about how to operationally define the term, one commonality is that cognitive flexibility is a component of executive functioning. Research has primarily been conducted with children at the school age; however, individual differences in cognitive flexibility are apparent across the lifespan. Measures for cognitive flexibility include the A-not-B task, Dimensional Change Card Sorting Task, Multiple Classification Card Sorting Task, Wisconsin Card Sorting Task, and the Stroop Test. Functional Magnetic Resonance Imaging (fMRI) research has shown that specific brain regions are activated when a person engages in cognitive flexibility tasks. These regions include the prefrontal cortex (PFC), basal ganglia, anterior cingulate cortex (ACC), and posterior parietal cortex (PPC). Studies conducted with people of various ages and with particular deficits have further informed how cognitive flexibility develops and changes within the brain. Cognitive flexibility also has implications both inside and outside of the classroom. A person’s ability to switch between modes of thought and to simultaneously think about multiple concepts has been shown to be a vital component of learning.