The common law provides for a defence of necessity for "emergency situations where normal human instincts, whether of self‑preservation or of altruism, overwhelmingly impel disobedience." The defence provides a legal excuse (as opposed to a justification) for conduct making out the offence.
The defence is to be "strictly controlled and scrupulously limited" to situations of true involuntariness.
Burden of Proof
The accused has the burden to establish an air of reality that the defence applies. This requires at least some evidence that all the elements of necessity may be satisfied. Once established the burden moves to the Crown to establish beyond a reasonable doubt that at least one of the elements of the defence does not apply.
The elements to make out the defence of necessity requires proof that:
- the accused must be in imminent peril or danger;
- the accused must have had no reasonable legal alternative to the course of action he or she undertook; and
- the harm inflicted by the accused must be proportional to the harm avoided by the accused.
The elements of imminent peril and no alternative is determined on a modified objective standard taking into account the situation and characteristics of the accused. The element of proportionality is measured on an objective standard.
- See Morgentaler v The Queen, 1975 CanLII 8 (SCC),  1 SCR 616, at p. 678
- R v Latimer, 2001 SCC 1 (CanLII),  1 SCR 3 at 32 to 34 (“The accused person must, at the time of the act, honestly believe, on reasonable grounds, that he faces a situation of imminent peril that leaves no reasonable legal alternative open.”)
Latimer, ibid. at para 34
Imminent Peril or Danger
The peril or danger must be more than just foreseeable or likely. It must be near and unavoidable. At the least, "the situation must be so emergent and the peril must be so pressing that normal human instincts cry out for action and make a counsel of patience unreasonable."
The consequence must be "imminent or harm unavoidable and near... It must be on the verge of transpiring and virtually certain to occur."
- R v Perka, 1984 CanLII 23 (SCC),  2 SCR 232
R v Latimer, 2001 SCC 1 (CanLII) at para 29
No Reasonable Legal Alternative
The question of reasonableness turns on the particular circumstances.
The inquiry in the second requirement focuses on whether the accused had any real choice. Where he has no real choice then his actions are effectively not "morally voluntary".
The accused "need not be placed in the last resort imaginable, but he must have no reasonable legal alternative."
The judge need not consider every potential possibility in hindsight.
In an impaired driving case, it has been accepted that the accused need not knock on the doors of strangers late at night during a storm to find someone who would drive them.
In considering this element on an impaired charge. The harm associated with the offence is grave. The fact that no one is hurt is of little importance. The comparator harm avoided should relate to the "preservation of life".
Latimer at para 31
Latimer at para 31
R v Desrosiers,  OJ No 1985 (ONCJ) per Keast J. at para 31
R v Costoff,  OJ 1261 per Bourque J at para 29
Specific Types of Offences
Necessity was not available for an accused who burned a highway bridge in order to draw attention to its need for repair as there were reasonable alternatives.
- R v Stevenson,  5 W.W.R. 737, 42 Man.R. (2d) 133 (Man. Q.B.)(*no link) leave denied,  1 W.W.R. 767 (Man. C.A.)
Where the accused or someone in his protection is at "immediate risk of physical harm, if no reasonable alternative is available and, if the driving is for no longer than is necessary to escape the harm, the defence of necessity will succeed".
Necessity no longer applies where the driver drove longer than is necessary in the circumstances.
- R v L.S., 2001 BCPC 462 (CanLII),  BCJ No. 3062 at para 25
R v Drake, 1998 CarswelMan 232 (Prov.Ct.)(*no link): the accused drove past a police station, hospital and gas station
R v Brown, 1998 CarswellOnt 788 (Sup.Ct.Just.)(*no link): drove past a police station
R v Murray, 2010 ABQB 784 (CanLII) at para 33-35
Necessity is a potential defence for dangerous driving causing death where the accused had reason to flee from gunshots that were putting his life at risk.
- Primus c. R., 2010 QCCA 1541 (CanLII)
Break and Enter
Necessity has been considered in relation to break and enter. 
R v John Doe, 2007 BCCA 490 (CanLII),  BCJ No. 2111, 228 CCC (3d) 302 (BCCA) - new trial ordered for failure to consider all factors. Accused broke into house to be warm and get food
Fraud and Related
A woman was found not guilty of fraud for failing to report her co-habitation to social services due to suffering from battered wife syndrome and believed she had no other choice.  Battered wife syndrome was not successful in a necessity defence for possession and laundering proceeds of crime.
False pretenses not available where there are other options available.