What are lessons learned and when should they be captured?

Lessons learned are the knowledge gained during a project about what went well, what went poorly, and what should be done differently in the future. They should be captured throughout the project at the end of each phase, after significant events, and during the closing process group, not only at the very end. Lessons learned sessions involve the project team reviewing successes, failures, and improvement opportunities. The documented lessons are stored in the organizational process assets or lessons learned repository so future projects can benefit from past experience.