The planning fallacy is a phenomenon in which predictions about how much time will be needed to complete a future task display an optimism bias and underestimate the time needed. This phenomenon sometimes occurs regardless of the individual's knowledge that past tasks of a similar nature have taken longer to complete than generally planned. The bias affects predictions only about one's own tasks; when outside observers predict task completion times, they tend to exhibit a pessimistic bias, overestimating the time needed. The planning fallacy involves estimates of task completion times more optimistic than those encountered in similar projects in the past.

The planning fallacy was first proposed by Daniel Kahneman and Amos Tversky in 1979. In 2003, Lovallo and Kahneman proposed an expanded definition as the tendency to underestimate the time, costs, and risks of future actions and at the same time overestimate the benefits of the same actions. According to this definition, the planning fallacy results in not only time overruns, but also cost overruns and benefit shortfalls.