Workers and organizations routinely choose the complexity of tasks that they undertake. We study how cognitive ability and overconfidence shape this choice of task complexity, both theoretically and experimentally. Consistent with our model, we find that more cognitively able individuals choose more complex tasks. Conditional on cognitive ability, we find that more overconfident individuals also choose more complex tasks. About 80% of subjects pick their payoff-maximizing task, and higher cognitive ability predicts more efficient choices of task complexity. But errors are strikingly one-sided: nearly all who fail to choose optimally pick tasks that are too complex. Our findings suggest that organizations should pair task menus and incentives for choosing more complex tasks with information and feedback that help workers assess their ability and probability of success. This preserves the benefits of self-selection while reducing costly overreach.