The lessons learned process: what it means, how to run it, and why most lessons are never learned
What lessons learned means in project management, how to run a review step by step, how to write a lesson, and why most lessons never change practice.
In project management, lessons learned are "documented experiences that can be used to improve the future management of projects, programmes and portfolios". That is the definition of the Association for Project Management (APM), the UK's professional body for project management. The lessons learned process is the way a team collects these experiences after a project, an incident or a crisis, and turns them into changes. French organisations call the same practice retour d'expérience, or REX.
The word "learned" is the hard part. NATO's handbook on the subject puts it simply: a lesson is not learned until something changes in the way people work. This guide explains the terms, the steps of a lessons learned review, how to write a lesson that others can use, and why so many lessons stay in a report.
Lessons learned, after-action review, post-mortem: the terms
Several kinds of review collect lessons. They differ in timing, in who takes part and in what they produce.
| When | Who | What it produces | |
|---|---|---|---|
| After-action review | during or right after an event | the people involved, with a facilitator | what was supposed to happen, what happened, why, what to keep and what to improve |
| Post-mortem (post-incident review) | after an incident, or at the end of a project | the team involved | a timeline, the causes and corrective actions |
| Sprint retrospective | at the end of each sprint, in Scrum | the Scrum team | improvements for the next sprint |
| Lessons learned review | at the end of a project phase, a project or a crisis | the team, often with people from outside it | lessons, actions with owners, and a record other teams can reuse |
The first three mostly serve the team that just did the work. A lessons learned review also serves people who were not there: the next project team, another site, a new joiner.
Two more terms come from military and NATO practice. Most of this guide depends on the difference between them. In the NATO Lessons Learned Handbook (fourth edition, 2022), a lesson identified is an observation that has been analysed: the root cause is known, and a corrective action and someone with the authority to order it have been named. A lesson learned is the improvement that results once that action is complete and, where needed, checked. The US Army defines a lesson learned the same way in its regulation on its lessons learned programme (2022): a change to a capability, a process or a procedure.
Where the practice comes from
The after-action review comes from the US Army. Its current guide, TC 7-0.1 After Action Reviews (February 2025), defines it as a guided analysis of how a unit performed. The review is held during or right after an event, to improve future performance. The guide also states that an after-action review "is not a critique". The World Health Organization (WHO) adapted the method for health emergencies in its guidance for after action review (2019).
NASA has collected lessons in a central system since 1994. In France, safety-critical industries use the same practice under the name retour d'expérience: the environment ministry has run ARIA, a public database of industrial accidents and incidents, since 1993.
The lessons learned process, step by step
NATO's handbook splits the process into two phases. The analysis phase turns observations into lessons identified. The implementation phase turns them into lessons learned. The steps below follow that handbook, the US Army's guide to after-action reviews, the WHO guidance and the APM:
- Plan the reviews at the start. The APM advises creating a lessons learned log when the project starts, and holding a review at the end of each formal phase. If you wait for the end of the project, most of the team is already working on the next one.
- Collect observations while they are fresh. The US Army holds its reviews during or immediately after each event. Record what happened before people start to explain it.
- Hold the review. The US Army's agenda covers what was supposed to happen, what happened, the key issues, and what to sustain or improve. Do not look only at problems: the WHO asks what went well and what went less well.
- Find the root cause. In NATO's definition, an observation becomes a lesson identified only when its root cause is established.
- Decide the actions, with an owner and a deadline. The WHO calls the action plan the main output of a review: each action has a person responsible and a timeline.
- Complete the actions and check them. NATO calls this step "implement and validate". The WHO follows up on the action plan three months after the review, then every quarter.
- Share the lesson. Other teams can only reuse a lesson they can find.
The meeting itself works well. Two meta-analyses of debriefs, the general name for these reviews, reach the same conclusion. Tannenbaum and Cerasoli (2013) found that well-run debriefs improve performance by about 20 to 25%. Keiser and Arthur (2021) found an even larger effect across 61 studies. Most lessons are lost in steps 5 to 7.
How to write a lesson
NATO writes each observation in four parts: observation, discussion, conclusion, recommendation. A lessons learned record that others can use usually contains:
- the context and the original objective;
- what was supposed to happen, and what happened;
- the root cause;
- what worked, and should be kept;
- the lesson, in one sentence;
- the action, with an owner, a deadline and how you will check the result;
- keywords, so people can find the lesson later.
A few writing rules follow from the sources above. Write one lesson per record. Write it as an instruction someone can act on: "Book the test environment four weeks before go-live" helps the next team, "Planning could be improved" does not. Describe causes, not people: NATO lists the fear of embarrassment and blame among the common barriers to sharing a lesson. Keep the facts separate from your interpretation of them.
Our lessons learned template and after-action review template follow this structure.
Why most lessons are never learned
According to the APM, many organisations report that lessons may be documented, but they are "not reliably learned and practices adjusted". Public audits and research show where the process breaks:
- Lessons are stored, then not used. In 2012, NASA's Office of Inspector General surveyed 28 project managers about the agency's lessons learned system. Of these, 16 (57%) used it, and 12 (43%) contributed to it. The managers said the system was outdated, hard to use and short on information relevant to their projects.
- Nobody checks that the action worked. A Dutch study of how organisations learn from incidents (Drupsteen and colleagues, 2013) found that the learning potential was used least in the evaluation stage, where organisations check whether their measures worked.
- People think recording is enough. NATO's handbook lists this belief among the common barriers, next to lessons that are hard to find. Without a decision on the corrective action and on who is responsible for it, the handbook says, lessons identified "will likely stall".
Three practices from the same sources reduce the loss:
- Change the document people already use. A lesson that updates a standard operating procedure reaches everyone who follows that procedure, including people who never read the review.
- Make the lesson available when it is needed. NATO's handbook says the process only has value when the information reaches the people who need it, when they need it.
- Give the follow-up an owner. The US Army's guide states that the leaders' actions after a review are "nearly as important" as the review itself.
Some experience is hard to document. It passes from person to person, through knowledge transfer and communities of practice.
What our customers do
Quantis, a sustainability consultancy, delivers around 800 projects a year. Its teams used to finish a project and start the next one, with no standard debrief. Now each team fills in an executive summary from a template when a project closes, and the firm's knowledge management specialist tracks project end dates so the request arrives at the right moment. Quantis went from around 20 project debriefs in 2022 to 300 two years later, and 89% of its users find knowledge through search. This is what we build with Elium: lessons written from a template, each with an owner and a review date, that the next team finds when it needs them.
Common questions
The APM defines lessons learned as documented experiences that can be used to improve the future management of projects, programmes and portfolios. The term also names the process: collecting these experiences during and after a project and turning them into changes.
A lesson identified has been analysed: its root cause is known and a corrective action is proposed. It becomes a lesson learned only when that action has been completed and has improved the way people work. The definitions come from NATO and the US Army.
If nothing has changed yet, "lessons identified" is the more accurate term. For the meeting itself, people say after-action review, post-project review, post-mortem or retrospective. In French, the practice is called retour d'expérience (REX).
The US Army's guide covers what was supposed to happen, what happened, the key issues, and what to sustain or improve. The WHO adds what went well and what went less well.
One lesson per record, with the context, what happened, the root cause, the lesson in one sentence, and an action with an owner and a deadline. NATO writes each one in four parts: observation, discussion, conclusion, recommendation.
Yes. It is the standard term in project management and in military practice: the APM, NATO and the US Army all use it.
Where to start
Open your last lessons learned report. For each lesson, find the document it was meant to change, and check that it changed. If you cannot find the document or the person responsible, start there. To see how Elium links each lesson to an owner and a review date, book a demo.
Sources
- Association for Project Management, What is lessons learned in project management?
- NATO Joint Analysis and Lessons Learned Centre, The NATO Lessons Learned Handbook, fourth edition, June 2022.
- US Army, TC 7-0.1, After Action Reviews, 13 February 2025, and AR 11-33, Army Lessons Learned Program, 2022.
- World Health Organization, Guidance for after action review (AAR), 2019.
- NASA Office of Inspector General, Review of NASA's Lessons Learned Information System, IG-12-012, 6 March 2012.
- French Ministry for the Ecological Transition, BARPI, The ARIA database (in French).
- Drupsteen, L., Groeneweg, J. and Zwetsloot, G. (2013), Critical steps in learning from incidents, International Journal of Occupational Safety and Ergonomics.
- Tannenbaum, S. and Cerasoli, C. (2013), Do team and individual debriefs enhance performance? A meta-analysis, Human Factors.
- Keiser, N. and Arthur, W. (2021), A meta-analysis of the effectiveness of the after-action review (or debrief), Journal of Applied Psychology.
- Ken Schwaber and Jeff Sutherland, The Scrum Guide, November 2020.