Legal immunity, or immunity from prosecution, is a legal status wherein an individual or entity cannot be held liable for a violation of the law, in order to facilitate societal aims that outweigh the value of imposing liability in such cases. Such legal immunity may be from criminal prosecution, or from civil liability (being subject of lawsuit), or both. The most notable forms of legal immunity are diplomatic immunity, judicial immunity, and witness immunity. One author has described legal immunity as "the obverse of a legal power"

A party has an immunity with respect to some action, object, or status, if some other relevant party – in this context, another state or international agency, or citizen or group of citizens – has no (power) right to alter the party's legal standing in point of rights or duties in the specified respect. There is a wide range of legal immunities that may be invoked in the name of the right to rule. In international law, immunities may be created when states assert powers of derogation, as is permitted, for example, from the European Convention on Human Rights "in times of war or other public emergency." Equally familiar examples include the immunities against prosecution granted to representatives (MPs or councilors) and government officials in pursuit of their duties. Such legal immunities may be suspected as potential violations of the rule of law, or regarded as quite proper, as necessary protections for the officers of the state in the rightful pursuit of their duties.

In criminal law, diminished responsibility (or diminished capacity) is a potential defense by excuse by which defendants argue that although they broke the law, they should not be held fully criminally liable for doing so, as their mental functions were "diminished" or impaired.

Diminished capacity is a partial defense to charges that require that the defendant act with a particular state of mind. For example, if the felony murder rule does not apply, first degree murder requires that the state prove beyond a reasonable doubt that the defendant acted with premeditation, deliberation, and the specific intent to kill—all three are necessary elements of the state's case. If evidence exists, sufficient to create a reasonable doubt as to whether the defendant because of mental illness or "defect" possessed the capacity to premeditate, deliberate or form the specific intent to kill then the state cannot convict the defendant of first degree murder. This does not mean that the defendant is entitled to an acquittal. The defendant still might be convicted of second degree murder which only requires that the defendant act with general malice.

The defense's acceptance in American jurisdictions varies considerably. The majority of states have adopted it by statute or case decision, and a minority even recognize broader defenses such as "irresistible impulse". Some U.S. states restrict the defense to the charge of murder only where a successful defense will result in a manslaughter conviction instead of murder. Until recently, the Republic of Ireland did not accept the partial defense. The Irish Supreme Court had rejected the existence of the defense in DPP v O'Mahony. The case was recently abrogated, however, by enactment of the Criminal Law (Insanity) Act 2006, effective June 1, 2006. The act, in pertinent part, specifically adopted the partial defense for the charge of murder where a successful defense will result in a manslaughter conviction instead of murder.