Peacekeeping by the United Nations is a role of the United Nations's Department of Peace Operations and an "instrument developed by the organization as a way to help countries torn by conflict to create the conditions for lasting peace". It is distinguished from peacebuilding, peacemaking, and peace enforcement although the UN does acknowledge that all activities are "mutually reinforcing" and that overlap between them is frequent in practice.

Peacekeepers monitor and observe peace processes in post-conflict areas and assist ex-combatants in implementing the peace agreements they may have signed. Such assistance comes in many forms, including separating former combatants, confidence-building measures, power-sharing arrangements, electoral assistance, strengthening the rule of law, and economic and social development. Accordingly, UN peacekeepers (often referred to as Blue Berets or Blue Helmets because of their light blue headgear) can include soldiers, police officers, and civilian personnel.

Chapter VII of the United Nations Charter gives the United Nations Security Council the power and responsibility to take collective action to maintain international peace and security. Most of these operations are established and implemented by the United Nations itself, with troops obeying UN operational control. In these cases, peacekeepers remain members of their respective armed forces, and do not constitute an independent "UN army", as the UN does not have such a force. In cases where direct UN involvement is not considered appropriate or feasible, the Council authorizes regional organizations such as NATO, the Economic Community of West African States, or coalitions of willing countries to perform peacekeeping or peace-enforcement tasks.

Jean-Pierre Lacroix is the Head of the Department of Peace Operations; he took over from the former under-secretary-general Hervé Ladsous on 1 April 2017. Since 1997, all directors have been French. DPKO's highest level doctrine document, entitled "United Nations Peacekeeping Operations: Principles and Guidelines" was issued in 2008.

Once a peace treaty has been negotiated, the parties involved might ask the United Nations for a peacekeeping force to supervise various elements of the agreed upon plan. This is often done because a group controlled by the United Nations is less likely to favor the interests of any one party since it itself is controlled by many groups, namely the 15-member Security Council and the intentionally diverse United Nations Secretariat.

If the Security Council approves the creation of a mission, then the Department of Peacekeeping Operations begins planning for the necessary elements. At this time, the senior command team is selected. The department will then seek contributions from member nations. Since the UN has no standing force or supplies, it must form ad hoc coalitions for every task undertaken. Doing so results in both the possibility of failure to form a suitable force, and a general slowdown in procurement once the operation is in the field. Roméo Dallaire, force commander in Rwanda during the Rwandan genocide there, described the problems this poses by comparison to more traditional military deployments:

He told me the UN was a "pull" system, not a "push" system like I had been used to with NATO, because the UN had absolutely no pool of resources to draw on. You had to make a request for everything you needed, and then you had to wait while that request was analyzed... For instance, soldiers everywhere have to eat and drink. In a push system, food and water for the number of soldiers deployed is automatically supplied. In a pull system, you have to ask for those rations, and no common sense seems to ever apply.

— (Shake Hands With the Devil, Dallaire, pp. 99–100)