In recent years, IT project failures have received a great deal of attention in the press as well as the boardroom. In an attempt to avoid disasters going forward, many organizations are now learning from the past by conducting retrospectives—that is, project postmortems or post-implementation reviews. While each individual retrospective tells a unique story and contributes to organizational learning, even more insight can be gained by examining multiple retrospectives across a variety of organizations over time. This research aggregates the knowledge gained from 99 retrospectives conducted in 74 organizations over the past seven years. It uses the findings to reveal the most common mistakes and suggest best practices for more effective project management.