A conscientious objector is an individual whose personal beliefs are incompatible with military service, or sometimes with any role in the armed forces. This constitutes a conflict in the case of conscription.

The reasons for refusing to serve are varied. Many conscientious objectors are so for religious reasons -- notably, the Quakers are pacifist by doctrine and Jehovah's Witnesses, who, while not strictly speaking pacifists, refuse to participate in the armed services on the grounds that they believe Christians should be neutral in worldly conflicts. Other objections can stem from a deep sense of responsibility toward humanity as a whole, or from simple denial that any government should have that kind of moral authority.

The legal status of conscientious objectors has varied over the years and from nation to nation. Many conscientious objectors have been imprisoned for refusing to participate in wars. In the United States, the Supreme Court ruled in 1970 that it is not necessary for a conscientious objector to have a religious basis for their beliefs.

Because of their conscientious objection to participation in military service, whether armed or unarmed, Jehovah's Witnesses have often faced imprisonment or other penalties. In Greece, for example, before the introduction of alternative civilian service in 1997, hundreds of Witnesses were imprisoned, some for three years or even more for their refusal. More recently, in Armenia, young Jehovah's Witnesses have been imprisoned (and remain in prison) because of their conscientious objection to military service.

Some conscientious objectors are unwilling to serve the military in any capacity, while others are willing to serve in non-combatant roles; in World War I, many conscientious objectors drove ambulances, often under fire. In World War II, some conscientious objectors volunteered for hazardous scientific experiments.

• See also: Conscientious objection throughout the world