A justification defense claims that the defendant's conduct was lawful under the circumstances, while an excuse defense concedes the wrongfulness of the act but argues the defendant should not be held criminally responsible. An example of justification is self-defense; an example of excuse is insanity.

The core elements of self-defense include an actual and reasonable belief that the use of force was necessary to prevent the imminent use of unlawful force by another. Deadly force is permissible only when the defendant reasonably believes they are facing an imminent threat of death or serious bodily injury.

The majority rule (and the Model Penal Code approach) does not require retreat before using deadly force unless the defendant knows they can avoid the threat with complete safety. The minority rule, or "stand your ground" rule, permits individuals to meet deadly force with deadly force, even if retreat is safely possible, as long as they are in a place where they have a legal right to be.

The defense of necessity allows a defendant to commit a crime to avoid a greater imminent harm when no adequate legal alternative is available. Two key limitations are that the defense is generally not available if the defendant caused the situation and is typically not a defense to homicide.

The M’Naghten Rule states that a defendant is not guilty by reason of insanity if, due to a mental disease or defect, they did not know the nature and quality of their act, or if they did know it, they did not know it was wrong. The central focus of this test is the defendant's cognitive understanding of their actions and their wrongfulness.

Voluntary intoxication, the result of the defendant's intentional consumption of intoxicants, may negate the specific intent required for certain crimes. Involuntary intoxication, which occurs through coercion, fraud, or mistake, is treated more like insanity and may be a defense to any crime if it negates the necessary mental state.

Duress can be a valid defense if the defendant committed a crime under the threat of imminent death or serious bodily injury to themselves or others, the threat would overcome the will of a person of ordinary firmness, and there was no reasonable opportunity to escape. It is generally not a defense to homicide.

The subjective test for entrapment focuses on the predisposition of the defendant to commit the crime. If the defendant was already inclined to commit the crime and the government merely provided an opportunity, the defense of entrapment will likely fail, regardless of the government's conduct.

The general rule is that mistake of law is not a defense; ignorance of the law is no excuse. One recognized exception is when the defendant reasonably relied on an official interpretation of the law by a public official responsible for interpreting or enforcing that law (entrapment by estoppel).

Due process guarantees fundamental fairness in the criminal process, which includes the right of a defendant to present a defense. This means defendants must have the opportunity to introduce evidence, challenge the prosecution's case, and have their defenses properly considered by the court and jury.