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Abstract: The aim of the current study was to explore the effects of problem structure, namely goal hierarchy and number of optimal solution paths, on the neural architecture that supports problem-solving and planning. Here, six-move problems with both an unambiguous and ambiguous goal hierarchy and single and multiple optimal solution paths were examined. In the task used, participants were encouraged to generate a solution plan before execution. The behavioral results revealed that problem-solving time and accuracy were both affected by both problem parameters. The fMRI activation results revealed three major findings. First, the right prefrontal cortex revealed a significantly different activation pattern than the other regions examined. This was the only region that revealed a larger response during the execution phase than the planning phase. Second, the effect of goal hierarchy was strongest during the execution phase. Finally, while there was no main effect of number of optimal solution paths, this parameter interacted with goal hierarchy in a number of regions across the brain. The present study also suggests that the minimum number of moves may not be the best measure of problem difficulty and that greater care be taken in the selection of TOL problems for both experimental studies as well as clinical assessment. [Copyright &y& Elsevier]