Self-defense is a justification defense when a non-aggressor uses force to defend himself. It requires an honest and reasonable belief that the use of force was necessary to defend against the immediate use of unlawful force and that the force used was reasonably necessary in the circumstances.
The use of force must not be excessive in relation to the harm threatened.
At common law, deadly force is never permitted to repel a non-deadly attack, nor may it be used if a non-deadly response will apparently suffice. The Model Penal Code allows it to protect against immediate death, serious bodily injury, forcible rape, or kidnapping.
The Model Penal Code instead requires that the force be "immediately necessary" to permit force when required to repel the attack instead of when the threat is immediate.
At common law, a person is required to retreat if he can do so in complete safely. However, a slight majority of jurisdictions have enacted stand your ground laws, removing the duty to retreat. In addition, all jurisdictions have an exception to the duty to retreat if one is attacked in his own dwelling place or it curtilage (immediately surrounding land). Some jurisdictions expand this to one's vehicle, and the MPC expands it to one's place of work.