Parties to offence
21. (1) Every one is a party to an offence who
- (a) actually commits it;
- (b) does or omits to do anything for the purpose of aiding any person to commit it; or
- (c) abets any person in committing it.
(2) Where two or more persons form an intention in common to carry out an unlawful purpose and to assist each other therein and any one of them, in carrying out the common purpose, commits an offence, each of them who knew or ought to have known that the commission of the offence would be a probable consequence of carrying out the common purpose is a party to that offence.
R.S., c. C-34, s. 21.
Section 21 outlines four ways in which a person can be criminally liable for an act. A person can be a principal, an aider, an abettor, or have common intention to commit an offence.
In any of the circumstances, a party to an offence must have both knowledge and intent.
Where a person provides directions or instructions to a potential buyer to make a purchse of drugs from a seller, that can amount to aiding and abetting in trafficking arising from the eventual sale. 
Section 21(1)(a): CommitsEdit
A person "actually commits" an offence when he does some act "towards the commission of the offence" with requisite mens rea or uses an agent to commit it.
This means that where there is multiple people all doing some act together towards the shared achievement of the offence, each is actually committing the offence as a "joint principle offender".
This means that where it is proven that where it is proven that multiple people acted with an intention to commit murder, it is "legally irrelevant" to determine who pulled the trigger.
Section 21(1)(b), (c): Aiding and AbettingEdit
The actus reus of aiding or abetting is "doing (or, in some circumstances, omitting to do) something that assists or encourages the perpetrator to commit the offence. While it is common to speak of aiding and abetting together, the two concepts are distinct, and liability can flow from either one. Broadly speaking, "[t]o aid under s. 21(1)(b) means to assist or help the actor... . To abet within the meaning of s. 21(1)(c) includes encouraging, instigating, promoting or procuring the crime to be committed"
Section 21(2): Common IntentionEdit
A common intention is whether two or more persons "have in mind the same unlawful purpose." The common intention may form "in the instant of the offence being committed, the mutual intention to pursue unlawful purpose and to assist each other therein being formed at the very moment of carrying it out." 
So for example, where a second party joins in on an assault by a primary party, there will be a common intention formed.
A girlfriend of the principal was an abettor of first degree murder under s. 21(1)(c) by standing by during the murder and yelled "kill him Georgie".
The girlfriend of a principal was an abettor of manslaughter where she gave the principal a weapon for the purpose of attack the victim. However, without the formed intent in giving the weapon, there will be no conviction. Unless there is some duty to act, a bystander who is present and watches a murder cannot be found guilty of any offence connected to the murder.
Person counselling offence
22. (1) Where a person counsels another person to be a party to an offence and that other person is afterwards a party to that offence, the person who counselled is a party to that offence, notwithstanding that the offence was committed in a way different from that which was counselled.
(2) Every one who counsels another person to be a party to an offence is a party to every offence that the other commits in consequence of the counselling that the person who counselled knew or ought to have known was likely to be committed in consequence of the counselling.
Definition of “counsel”
(3) For the purposes of this Act, “counsel” includes procure, solicit or incite.
R.S., 1985, c. C-46, s. 22; R.S., 1985, c. 27 (1st Supp.), s. 7.
"Counsel" under this section is more that simply advising, it has the "meaning of actively inducing" (R v Sharpe, 2001 SCC 2)
The mens rea of counselling requires evidence that “an accused either intended that the offence counseled be committed, or knowingly counseled the commission of the offence while aware of the unjustified risk that the offence counseled was in fact likely to be committed as a result of the accused’s conduct" 
Where a threat allegedly counsels murder the test to be applied is whether 1) an ordinary person would view the statement objectively would take it as an invitation to kill and 2) the accused either intended or knowingly counselling the victim's murder while aware of the unjustified risk that murder would likely be committed. 
Where offence is not committedEdit
Counselling offence that is not committed
464. Except where otherwise expressly provided by law, the following provisions apply in respect of persons who counsel other persons to commit offences, namely,
- (a) every one who counsels another person to commit an indictable offence is, if the offence is not committed, guilty of an indictable offence and liable to the same punishment to which a person who attempts to commit that offence is liable; and
- (b) every one who counsels another person to commit an offence punishable on summary conviction is, if the offence is not committed, guilty of an offence punishable on summary conviction.
R.S., 1985, c. C-46, s. 464; R.S., 1985, c. 27 (1st Supp.), s. 60.
Organizations as PartiesEdit
Offences of negligence — organizations
22.1 In respect of an offence that requires the prosecution to prove negligence, an organization is a party to the offence if
- (a) acting within the scope of their authority
- (i) one of its representatives is a party to the offence, or
- (ii) two or more of its representatives engage in conduct, whether by act or omission, such that, if it had been the conduct of only one representative, that representative would have been a party to the offence; and
- (b) the senior officer who is responsible for the aspect of the organization’s activities that is relevant to the offence departs — or the senior officers, collectively, depart — markedly from the standard of care that, in the circumstances, could reasonably be expected to prevent a representative of the organization from being a party to the offence.
2003, c. 21, s. 2.
Other offences — organizations
22.2 In respect of an offence that requires the prosecution to prove fault — other than negligence — an organization is a party to the offence if, with the intent at least in part to benefit the organization, one of its senior officers
- (a) acting within the scope of their authority, is a party to the offence;
- (b) having the mental state required to be a party to the offence and acting within the scope of their authority, directs the work of other representatives of the organization so that they do the act or make the omission specified in the offence; or
- (c) knowing that a representative of the organization is or is about to be a party to the offence, does not take all reasonable measures to stop them from being a party to the offence.
2003, c. 21, s. 2.
Cases on PartiesEdit
- R. v. Briscoe, 2012 ABQB 239 -- accused not party to a murder
- R. v. Opio, 2011 ABPC 392 -- guilty as party to trafficking
- R. v. Mohamed, 2011 ABCA 350 || convicted as driver; D argued that he was not aware of what was going on
- R. v. Crowchild, 2011 ABPC 299 -- guilty aggravated assault; not guilty robbery
- R. v. Laurencelle, 1999 BCCA 511
- R. v. Sparrow (1979) 51 C.C.C. (2d) 443, 12 C.R. (3d) 158 (Ont. C.A.) -- guilty
- R. v. Thatcher  1 S.C.R. 652 -- guilty
- R. v. McMaster,  1 S.C.R. 740. -- guilty
- R. v. Dunlap and Sylvester,  2 S.C.R. 881.