In criminal law, automatism is a rarely used criminal defence. It is one of the mental condition defences that relate to the mental state of the defendant. Automatism can be seen variously as lack of voluntariness, lack of culpability (unconsciousness) or excuse. Automatism means that the defendant was not aware of their actions when making the particular movements that constituted the illegal act.

For example, in 1859, Esther Griggs threw her child out of a first floor window believing that the house was on fire, while having a sleep terror. In 2002, Peter Buck, lead guitarist of the band R.E.M., was cleared of several charges, including assault, which resulted from automatism brought on by a bad interaction between alcohol and sleeping pills. In a 2009 case in Aberporth in west Wales, Brian Thomas strangled his wife in their camper van, also during a sleep terror, when he mistook his wife for an intruder. The defence of automatism is denying that the person was acting in the sense that the criminal law demands. As such it is really a denial-of-proof – the defendant is asserting that the offence is not made out. The prosecution does not have to disprove the defence as is sometimes erroneously reported; the prosecution has to prove all the elements of the offence including the voluntary act requirement. Automatism is a defence even against strict liability crimes like dangerous driving, where no intent is necessary.

There are several limitations to the defence of automatism in English law. Prior fault generally excludes automatism. Intoxication generally excludes automatism, even when involuntary. Any defence that rests on insanity comes under the M'Naghten rules. Under English law internal causes of automatism are generally judged to be insane automatism and so result in the special verdict (not guilty by reason of insanity) rather than simple acquittal.

Automatism is arguably the only defence that excludes responsibility by negating the existence of the actus reus which uniquely allows it to be a defence to both conventional and strict liability offences (although this argument could be extended to the status defence of insanity, too). Strict automatism is a denial of actus reus and therefore most commonly used as a defence against strict liability offences. There are a number of reasons why a person may go into a state of automatism, including dissociation or hypo or hyperglycemia. Unconsciousness is the defence of denial of mens rea,^{[citation needed]} which is easier to prove and hence more commonly used for non-strict liability crimes. For example, in cases of homicidal sleepwalking the illegal act is typically not denied but the intent to kill is. The defendant will typically be perplexed and confused and will not cover up the episode. Kenneth Parks, after killing his mother-in-law and severely injuring his father-in-law, drove to the police station stating that he thought he had killed some people. The person's movements seem purposeful - the sleepwalker interacts with their environment in a limited way. Nonetheless the sleepwalker is not conscious of their actions. The use of the term "automatism" for these situations causes some confusion, as in these cases it is really the lack of intent on the part of the defendant which denies the mens rea of the offence rather than the actus reus (although this distinction is problematic in many instances), better called "unconsciousness". Intention is a problem in crimes of strict liability. Very few people intend to crash their vehicles, so clearly something better than intent is required to define automatism.

Another issue with automatism is that when the issue is raised by the defence as a realistic defence (an evidentiary basis), the prosecution then has to prove beyond reasonable doubt that the defendant was acting voluntarily. This is the case for several other defences e.g. duress. The justification for this is that voluntary action is part of the definition of the offence, and therefore something under the presumption of innocence the prosecution has to prove. The evidentiary burden was laid down in Hill v Baxter where the defence of automatism failed because there was no good evidence for the alleged blackout. Evidentiary burden means that the defendant needs to provide evidence to satisfy the judge that the issue should be put to the jury, which normally requires medical evidence (although R v Woolley, in which an HGV driver crashed after sneezing, proved an exception).

Because automatism is such a comprehensive defence, there are various exclusions to an automatism defence. The person must not be at fault. The classic example of this is falling asleep at the wheel of a car (Kay v Butterworth). Although one is not responsible for acts done while asleep, one can be held responsible for driving in a state where one would fall asleep at the wheel. The issue of prior fault applies to many diabetics who suffer hypoglycaemia while driving. Voluntary (and often involuntary) intoxication cannot cause legal automatism.

In many jurisdictions, there is a distinction made between "sane automatism" and "insane automatism". Where the involuntariness is caused by a mental illness, or "disease of the mind", as per the M'Naghten rules, it will be regarded as "insane automatism" and will often result in a special verdict of "not guilty by reason of insanity". This can have significant practical effects for the defendant, as they still may be detained after a special verdict as opposed to the straight acquittal available through sane automatism.

The M'Naghten rules require a "disease of mind", which requires an internal cause. This is medically nonsensical, and does not always bear much relationship to continuing risk which is the main justification. This means that insane automatisms do not require total loss of voluntary control (see below). Thus they are easier to prove in some circumstances, but conversely the burden of proof is on the defendant. Sleepwalking was initially an exception to the internal-external doctrine until the case of R v Burgess.