Prosecutorial discretion refers to the "discretion exercised by the Attorney-General in matters within his authority in relation to the prosecution of criminal offences”. It encompasses all "decisions regarding the nature and extent of the prosecution and the Attorney General's participation in it". The prosecution is empowered with discretionary authority over many decisions that have limited ability to be reviewed.
The discretion has its origins from the "Royal Prerogative of Justice and its enforcement in maintaining the King's Peace".
Discretion must be exercised "in relation to a specific case" and not simply a blanket choice of what laws to enforce.
Courts should be reluctant to interfere with prosecutorial discretion as they must respect the division of powers.
Discretion is essential to keep the system from becoming "unworkably complex and rigid."
Courts should not be reviewing the rationale for each and every decision of the Crown, it would be virtually impossible to have judge made rules to regulate the considerations involved. It would also create a conflict with the judge effectively regulating a prosecution.
The court cannot direct the Crown on what offences they must prosecute.
While the Attorney-General may enact policies that limit discretion, they are often considered undesirable.
Core matters of prosecutorial discretion cannot be reviewed except for abuse of process.
The conduct can be reviewed where the conduct of the Crown consituted a "marked and unacceptable departure from the reasonable standards expected of the prosecution"
Matters that have been found to be part of the core function of the Crown and so subject to prosecutorial discretion include:
- the decision to prosecute
- the stay of a charge
- the withdraw of a charge
- the acceptance of a lesser charge
- the Crown election
- taking control of a private prosecution
- the decision to appeal
- the decision to consent to an adjournment 
- consenting or refusal to consent to re-election
- notice to seek increased penalty
It is not constitutionally required that the Crown consider the aboriginal status of the accused before making decisions that will limit the judge's sentencing options.
The "existence of prosecutorial discretion does not offend the principles of fundamental justice".
R v Anderson, 2014 SCC 41 (CanLII) at para 44
R v Anderson, at para 44
Krieger v Law Society of Alberta, 2002 SCC 65 (CanLII),  3 SCR 372, at para 47
R v Smythe (1971), 1971 CanLII 30 (ON CA), 3 C.C.C.(2d) 97 (Ont. C.A.), per Wells, CJHC, at p. 105 and p. 109 aff'd at 1971 CanLII 831 (SCC)
R v Catagas, 1977 CanLII 1636 (MB CA),  38 CCC (2d) 296, per Freedman CJM, at p. 301
R v Power, 1994 CanLII 126 (SCC),  1 SCR 601
("courts have been extremely reluctant to interfere with prosecutorial discretion is clear from the case law. They have been so as a matter of principle based on the doctrine of separation of powers ...")
see also R v Cook, 1997 CanLII 392 (SCC),  1 SCR 1113
- R v Beare,  2 SCR 387, 1988 CanLII 126 (SCC) at p.410-411 per LaForest J.
R v Power, 1994 CanLII 126 (SCC),  1 SCR 601 at pp. 626-627
Power (“A judge does not have the authority to tell prosecutors which crimes to prosecute...”.)
- e.g. R v K.(M.), (1992) 74 CCC (3d) 108, 1992 CanLII 2765 (MB CA), at p. 110 - commenting on "zero-tolerance" policy on prosecution
See R v 974649 Ontario Inc., 2001 SCC 81 (CanLII),  3 SCR 575
R v L.L., 2015 ABCA 222 (CanLII), at para 10
- R v D.N., 2004 NLCA 44 (CanLII) at para 17
R v Power, 1994 CanLII 126 (SCC),  1 SCR 601 at para 41 to 43
R v Beare at pp. 410-411
- D.N. and R v Beare 1988 CanLII 126 (SCC),  2 SCR 387 at para 51
- R v Ng, 2003 ABCA 1 (CanLII)
- R v Gill, 2012 ONCA 607 (CanLII)
R v Anderson, at paras 29 to 33
R v Beare, 1988 CanLII 126 (SCC),  2 SCR 387, per La Forest J, at para 56
R v Lyons,  2 SCR 309, 1987 CanLII 25 (SCC), per La Forest J, at p. 348
R v Jones, 1986 CanLII 32 (SCC),  2 SCR 284, per La Forest J, at 303-304
There is no obligation on the Crown to call any witnesses at all, short of it amounting to abuse of process.
The decision to call a witness on a related subject after the Crown had determined the witness is otherwise unreliable can be acceptable in some circumstances.
Lemay v The King, 1951 CanLII 27 (SCC),  1 SCR 232, at p. 241
R v Jolivet, 2000 SCC 29 (CanLII),  1 SCR 751, at para 14
- Jolivet, ibid. at para 29
R v Rezaei, 2017 BCSC 611 (CanLII)
R v Cook, 1997 CanLII 392 (SCC),  S.C.J. No. 2, per L'Heureux-Dube J, at para 56
R v LL, 2015 ABCA 222 (CanLII)
Relationship with Police
The Crown and police are to consult with each other but the "maintenance of a distinct line between these two functions is essential to the proper administration of justice."
The Crown can be liable for their part in giving advice to police during an investigation.
While it is acceptable, the Crown should not try to be involved in interviews with parties prior to charges being laid.
Evaluating Whether to Proceed
Most prosecution offices are guided by Crown Policy manuals requirements that there be a "reasonable prospect of conviction" and there is a "public interest" in the proceeding.
One cannot evaluate the decisions of the Crown to proceed retroactively. The context of what was known at the time.
Considerations on the exercise of discretion must take into account the interest of the individual and the public interest.
Re-evaluation of proceedings can be based on aspects that comes to light at trial:
- witnesses may not testify in accordance with their earlier statements;
- weaknesses in the evidence may be revealed during cross-examination;
- scientific evidence may be proved faulty; or
- defence evidence may shed an entirely different light on the circumstances as they were known at the time process was initiated.
- Miazga v Kvello Estate,  3 SCR 339, 2009 SCC 51 (CanLII), at para 64
Miazga v Kvello Estate, ibid. at para 76
- R v Harrigan and Graham (1975), 33 C.R.N.S. 60 (Ont. C.A.), per Henry J, stated, at p. 69 ("In exercising these powers, which vitally concern the right and liberty of the individual, he must take into account not only the interest of the individual but also what the public interest requires.")
- Miazga v Kvello Estate, at para 76
The decision to proceed either by summary conviction or by imdictment is an "essential component of the fair and efficient operation of the criminal justice system".
R v Nur, 2013 ONCA 677 (CanLII), per Doherty JA, at para. 190, appealed to SCC on other issues at 2015 SCC 15 (CanLII)