A Do Not Resuscitate, or DNR order is a written order from a doctor that resuscitation should not be attempted if a person suffers cardiac or respiratory arrest. Such an order may be instituted on the basis of an advance directive from a person, or from someone entitled to make decisions on their behalf, such as a health care proxy; in some jurisdictions, such orders can also be instituted on the basis of a physician's own initiative, usually when resuscitation would not alter the ultimate outcome of a disease.
Any person who does not wish to undergo lifesaving treatment in the event of cardiac or respiratory arrest can get a DNR order, although DNR is more commonly done when a person who has an inevitably fatal illness wishes to have a more natural death without painful or invasive medical procedures.
It is commonly mistaken that a DNR means resuscitation would be successful. In contrast to medical dramas seen on TV, resuscitation in reality has only around a 10-percent success rate.[How to reference and link to summary or text] Even those who do survive, a suitably large proportion either die shortly afterward or are left with permanent physical/neurological deficiencies.[How to reference and link to summary or text] Therefore, medical students and doctors alike are being encouraged to be realistic with patients and their families when it comes to discussing such a sensitive topic as DNR orders, as it is thought that many wrongly believe resuscitation to be a risk-free guarantee to life.
Alternate namings and abbreviations for this order are used depending on the geographic region. DNR (Do Not Resuscitate) is a common abbreviation in the United States & the United Kingdom. It may be clarified in some regions with the addition of DNI (Do Not Intubate), although in some hospitals DNR alone will imply no intubation. Some areas of the United States and the United Kingdom include the letter A, as in DNAR, to clarify "Do Not Attempt Resuscitation." This alteration is so that it is not presumed by the patient/family that the attempt at resuscitation will be successful. Since the term DNR implies the omission of action, and therefore 'giving up', some have advocated for these orders to be re-termed Allow Natural Death. However the acronym AND could obviously be confused for the conjunction 'and' so this has not been popular in hospitals. New Zealand and Australia (and some hospitals in the UK) use the term NFR or Not For Resuscitation. Typically these abbreviations are written without periods in between the letters, i.e. DNR not D.N.R.
Until recently in the UK it was common to write "Not for 222" or conversationally, "Not for twos." This was implicitly a hospital DNR order, where 222 (or similar) is the hospital telephone number for the emergency resuscitation or crash team.
The DNR order came into being in the U.S. in the 1960s when defibrillation allowed the reversal of cardiac arrest, but this may prolong the life of the patient for only a short time.
In the U.S., cardiopulmonary resuscitation (CPR) and advanced cardiac life support (ACLS) should not be performed if a valid written "DNR" order is present. In any cases of doubt, emergency medical technicians, paramedics and other medical workers will perform as if a DNR order did not exist, as is required by law.
For the DNR to be valid there may be rules such as the use of a special form and/or additional signatures of a doctor and/or witnesses, etc. The exact rules for obtaining a DNR order, and for emergency medical personnel to accept the validity of one, vary widely according to jurisdiction. For example, in the state of Maryland, only state DNR orders are acceptable, and they require much verification, witnesses and doctor's signatures in order to be valid. In contrast, the state of Virginia allows patients to receive a DNR order with relative ease, and will accept them from most jurisdictions.
In many jurisdictions, medical professionals are ordered not to acknowledge DNR orders. This is especially common with paramedics, who are often employed by private companies unwilling to fight a potential lawsuit.
A DNR order must be filed on a jurisdictionally recognized form. A doctor contemplating a DNR decision should make every reasonable effort to confirm a DNR status before deciding to withhold resuscitation. To be sure that their DNR request is honored, patients should make their wishes clearly known to their family, doctors, community hospitals, care homes and any other caregivers or health care providers who may be present should a cardiac arrest occur. Some jurisdictions provide for a legally sanctioned DNR notification that is clearly visible to all healthcare personnel, similar to a MedicAlert bracelet.
A DNR order's specific effect depends on the hospital in which the incident occurs and the terms laid out in the DNR: neither cardiopulmonary resuscitation nor intubation will be performed, but treatment for infections or other treatable conditions, intravenous feeding and fluids, pain management and comfort care are generally continued.
In the United Kingdom, it has occurred that a hospital has gone against the wishes of a family to maintain a DNR order. 
-  UK Hospital fights to Retain "Do Not Resuscitate" Order Despite Child's Improvement
- Example of a DNR form
- Do Do-Not-Resuscitate orders endanger patients? Pro and Con arguments
- Decisions Relating to Cardiopulmonary Resuscitation Published by the Resuscitation Council (UK)