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Research on the cognitive processes underlying category learning provides evidence for two separate learning systems. A verbal system learns rule-defined (RD) categories and a nonverbal system learns non-rule-defined (NRD) categories. The objective of my dissertation is to explore the interaction between these systems. The verbal system is dominant in that adults tend to use it during initial learning but may switch to the nonverbal system when the verbal system is unsuccessful. The nonverbal system has traditionally been thought to operate independently of executive functions, but recent studies suggest that executive functions may be used to facilitate the transition away from the verbal system. Study 1 investigated whether executive functions play similar roles across systems and which, if any, components of executive functions are most important for the verbal and nonverbal systems. The components of executive functions were associated with both types of category learning but played different roles within each system. Study 2 compared the effects of a temporary and continuous executive function disruption for each system. When executive functions were continuously unavailable, the transition to the nonverbal system was hindered, providing evidence that executive functions are needed to transition between systems. For the verbal system, temporary and continuous interference had similar effects, illustrating that any executive function disruption is detrimental to the verbal system. Studies 3 and 4 experimentally manipulated the interaction between systems. Manipulating the order in which categories were learned affected the initial strengths of the systems. Strengthening the verbal system reduced optimal strategy use on subsequent nonverbal categorization, but the opposite was not true. Increasing stimulus knowledge facilitated rule searching and increased optimal strategy use on nonverbal categorization but not on verbal categorization. Conclusions. The current studies illustrate that the transition between systems is disrupted when executive functions are never fully available and when the verbal system is strengthened, but is facilitated when hypothesis testing is expedited. This research provides insight into the interaction between category learning systems and illustrates that the interaction is mediated by executive functions. Furthermore, executive functions play an important but different role in each system.