Summary executions have been practiced by police, military, and paramilitary organizations and are frequently associated with guerrilla warfare, counter-insurgency, terrorism, and any other situation which involves a breakdown of the normal procedures for handling accused prisoners, civilian or military.
In nearly all civilian jurisdictions, summary execution is illegal, as it violates the right of the accused to a fair trial. Almost all constitutions or legal systems based on common law have prohibited execution without the decision and sentence of a competent judge, and the UN's International Covenant on Civil and Political Rights (ICCPR) has declared the same:
Every human being has the inherent right to life. This right shall be protected by law. No man shall be deprived of his life arbitrarily.
In practice, though, extrajudicial killings have been performed by police and domestic forces in various countries and times, sometimes under martial law. It is also performed by armed bands fighting against governments and common citizens.
Under military law, summary execution is illegal in almost all circumstances, as a military tribunal would be the competent judge needed to determine guilt and declare a sentence of death. However, there are certain exceptions to this rule in emergencies and warfare where summary execution is legal.