Prediction markets allow participants to bet on the outcome of an event. Prediction markets, and prediction aggregators such as Metaculus, incentivize participants to make good predictions. When the prices of binary options in prediction markets are treated as probabilities, these probabilities sometimes closely match the real frequency of the predicted outcome. However, one limitation of prediction markets is that they cannot incentivize correct predictions where the predicted outcome prevents a payout. Because many believe that human-level AI might lead to human extinction, prediction markets may not work as well for AI timelines as for other questions.