(1). Duress by threat
Duress is an absolute defence which leads to an acquittal. It has requirements: (a). Nature of threat. Only threats of death or serious physical injury to the defendant (or certain others) will suffice.( Graham (1982 CA); Hasan (2005 HL)); (b).Immediacy: The requirement that the threat be immediate related to the notion that duress provided an inescapable pressure to offend but this has been broadened by judicial interpretation. The issue was not whether the threats could be carried out immediately but whether they were operating on the defendants at the time of the threat. ( R v. Hudson and Taylor  2 QB 202 (CA)); (c). Causal nexus: There must be a link between the threat made and the offence committed. Effectively, the person issuing the threat must nominate a particular offence to be committed.( R v. Cole  Crim LR 582 (CA))
In R v. Howe  1 AC 417 (HL), the House of Lords upheld the two-stage test outlined in Graham  1 WLR 294 (CA). This two-stage test requires not only that the defendant was overwhelmed by threats but also that a sober person of reasonable firmness would have been compelled to offend. Bowen  1 WLR 372 (CA) outlined three principles relevant to the attribution of the characteristics of the defendant to the reasonable man. The characteristics that were thought to be relevant were: age,sex, pregnancy, serious physical disability, recognised mental illness/psychiatric conditions.
Two factors that render duress unavailable:(a).voluntary association with a criminal gang. Just as duress is denied to a defendant who did not take a reasonable opportunity to escape, it is also withheld from a defendant who put himself in a position where he might be pressurised into offending.( R v. Sharp  QB 853 (CA)), in this situation, defendant should have foreseen or ought to have foreseen the risk of being subjected to compulsion to commit criminal offences(R v Hasan  HL); (b). commission of murder or attempted murder(Howe (1987 HL), Gotts  2 AC 412 (HL).).
(2). Duress of Circumstances
Duress of Circumstances uses same test and principles as duress by threats but where the compulsion comes from extraneous circumstances not humans (Hassan). The threat does not have to come from another person. It may come from the surrounding circumstances or from a naturally occurring event. And the offence is not nominated by another. The defendant commits an offence not because he is ordered to do so by another but because it seems to be the only way of avoiding the threat that he faces. It has several requirements: (a). The defendant’s actions must be a reasonable and proportionate response to a threat of death or serious injury. (Martin  1 All ER 652 (CA)); (b). The threat of death or serious injury must come from an external source. Threats emanating from the defendant will not suffice. (Rodger and Rose  1 Cr App R 143 (CA)); (c). The defendant’s conduct is only excused whilst the threat exists.( DPP v. Jones  RTR 33 (DC)).
4.Self-Defense and crime prevention
（Self-defence）Both self-defence and the use of defence in the prevention of crime are now covered by s 101A of Criminal Procedure Ordinance (Cap.221), but the principles that guide the operation of the defence were developed at common law. There are two requirements of self-defense:
(a),Defendant’s belief that force used is necessary. It is a subjective test. The necessity to use force is judged from the defendant’s perspective. A mistaken but honest belief does not exclude the availability of self-defense. (R v. Williams (Gladstone)  3 All ER 411 (CA)). However, case law has limited the availability of self-defence when the mistake about the need to use force was based on Mental illness and Consumption of drugs or alcohol. In Martin (Anthony)  QB 1 (CA), the defendant was not permitted to rely on self-defence following a mistake about the need to defend himself that was induced by his mental illness, which caused him to dramatically misinterpret events. In O’Grady  QB 995 (CA), the defendant was not entitled to rely on an intoxicated mistake about the need to use force to defend himself following the consumption of alcohol and hallucinogenic drugs.
(逃跑的可能)If the defendant had the opportunity to retreat but did not do so, it is not fatal to reliance on self-defence but defendant need to prove that he was not eager for a fight or that it was reasonable in the circumstances for him to stand his ground.( Bird  1 WLR 816
（预期防卫）Anticipatory self defence is permitted so long as it is reasonable (A-G’s Reference (No 2 of 1983) (1984 CA).
(b),Force used must be proportionate. It’s an objective test. This means that the reasonableness of the force will depend upon the circumstances. Two factors must be taken into account: (i).The level of force must be reasonable in response to the facts as the defendant believed them to be: Owino  2 Cr App R 128 (CA); (ii).Extreme situations can create pressure that distorts judgement: ‘a person defending himself cannot weigh to a nicety the exact measure of his necessary defensive actions’: Palmer  AC 814 (PC).
Self-defence justifies the force used so results in an outright acquittal if used successfully. There is no ‘half-way measure’ if, for example, the use of force was necessary but the level of force used was excessive: Clegg  1 AC 482 (HL).
（Prevention of crime）The defence of the prevention of crime under section 101A of Criminal Procedure Ordinance permits the use of reasonable force in the prevention of crime. This defence is wider than the common law defence of self-defence. The question here will be whether A was acting in the prevention of crime rather than out of revenge, and whether the degree of force he used was reasonable in the circumstances. If pleaded successfully, the prevention of crime will provide a complete defence to any charge of GBH and A will be acquitted.