This dissertation reports the results of two experiments that examined potential explanations for the underestimation of project completion time. Experiment 1 examined whether estimators pay attention to important task characteristics when they estimate project completion time. Surprisingly, the majority of the sample ignored the intercommunication among group members required by the task when they estimated project completion time. No expert-novice differences were found. These results show a serious deficiency in subjects' awareness of the effect of this task characteristic on project completion time. Experiment 2 examined whether people underestimate project completion time because they misaggregate probabilistic time estimates for project components. Indeed, this experiment found that people use heuristics to combine probability distributions for serial and parallel tasks. These heuristics cause people to underestimate project completion time. However, the magnitude of these errors was small, compared to that of Experiment 1. These results suggest that although the misaggregation of time estimates may contribute to the underestimation of project completion time, it is probably not a primary cause of this bias. In conclusion, the primary explanation for the underestimation of project completion time appears to be that estimators of project completion time ignore important task characteristics that affect project completion time. To our knowledge, these studies are the first experimental examinations of cognitive biases in the estimation of project completion time. Conclusions that can be drawn from these studies provide interesting hypotheses for future research.