Restitution - General. Kerr v. Baranow
In Kerr v. Baranow (SCC, 2011) the Supreme Court of Canada set out basics of unjust enrichment, here in 2011:
 The law of unjust enrichment has been the primary vehicle to address claims of inequitable distribution of assets on the breakdown of a domestic relationship. In a series of decisions, the Court has developed a sturdy framework within which to address these claims. .... Moore v. Sweet
B. The Legal Framework for Unjust Enrichment Claims
 At the heart of the doctrine of unjust enrichment lies the notion of restoring a benefit which justice does not permit one to retain: Peel (Regional Municipality) v. Canada, 1992 CanLII 21 (SCC),  3 S.C.R. 762, at p. 788. For recovery, something must have been given by the plaintiff and received and retained by the defendant without juristic reason. A series of categories developed in which retention of a conferred benefit was considered unjust. These included, for example: benefits conferred under mistakes of fact or law; under compulsion; out of necessity; as a result of ineffective transactions; or at the defendant’s request: see Peel, at p. 789; see, generally, G. H. L. Fridman, Restitution (2nd ed. 1992), c. 3-5, 7, 8 and 10; and Lord Goff of Chieveley and G. Jones, The Law of Restitution (7th ed. 2007), c. 4-11, 17 and 19-26.
 Canadian law, however, does not limit unjust enrichment claims to these categories. It permits recovery whenever the plaintiff can establish three elements: an enrichment of or benefit to the defendant, a corresponding deprivation of the plaintiff, and the absence of a juristic reason for the enrichment: Pettkus; Peel, at p. 784. By retaining the existing categories, while recognizing other claims that fall within the principles underlying unjust enrichment, the law is able “to develop in a flexible way as required to meet changing perceptions of justice”: Peel, at p. 788.
 The application of unjust enrichment principles to claims by domestic partners was resisted until the Court’s 1980 decision in Pettkus. In applying unjust enrichment principles to domestic claims, however, the Court has been clear that there is and should be no separate line of authority for “family” cases developed within the law of unjust enrichment. Rather, concern for clarity and doctrinal integrity mandate that “the basic principles governing the rights and remedies for unjust enrichment remain the same for all cases” (Peter v. Beblow, 1993 CanLII 126 (SCC),  1 S.C.R. 980, at p. 997).
 Although the legal principles remain constant across subject areas, they must be applied in the particular factual and social context out of which the claim arises. The Court in Peter was unanimously of the view that the courts “should exercise flexibility and common sense when applying equitable principles to family law issues with due sensitivity to the special circumstances that can arise in such cases” (p. 997, per McLachlin J. (as she then was); see also p. 1023, per Cory J.). Thus, while the underlying legal principles of the law of unjust enrichment are the same for all cases, the courts must apply those common principles in ways that respond to the particular context in which they are to operate.
In Moore v. Sweet (SCC, 2018) the Supreme Court of Canada considered the history and general principles underlying unjust enrichment (aka restitution):
A. Unjust Enrichment
 Broadly speaking, the doctrine of unjust enrichment applies when a defendant receives a benefit from a plaintiff in circumstances where it would be “against all conscience” for him or her to retain that benefit. Where this is found to be the case, the defendant will be obliged to restore that benefit to the plaintiff. As recognized by McLachlin J. in Peel (Regional Municipality) v. Canada, 1992 CanLII 21 (SCC),  3 S.C.R. 762, at p. 788, “At the heart of the doctrine of unjust enrichment . . . lies the notion of restoration of a benefit which justice does not permit one to retain.”
 Historically, restitution was available to plaintiffs whose cases fit into certain recognized “categories of recovery” — including where a plaintiff conferred a benefit on a defendant by mistake, under compulsion, out of necessity, as a result of a failed or ineffective transaction, or at the defendant’s request (Peel, at p. 789; Kerr, at para. 31). Although these discrete categories exist independently of one another, they are each premised on the existence of some injustice in permitting the defendant to retain the benefit that he or she received at the plaintiff’s expense.
 In the latter half of the 20th century, courts began to recognize the common principles underlying these discrete categories and, on this basis, developed “a framework that can explain all obligations arising from unjust enrichment” (L. Smith, “Demystifying Juristic Reasons” (2007), 45 Can. Bus. L.J. 281, at p. 281; see also Rathwell v. Rathwell, 1978 CanLII 3 (SCC),  2 S.C.R. 436, and Murdoch v. Murdoch, 1973 CanLII 193 (SCC),  1 S.C.R. 423, per Laskin J., dissenting). Under this principled framework, a plaintiff will succeed on the cause of action in unjust enrichment if he or she can show: (a) that the defendant was enriched; (b) that the plaintiff suffered a corresponding deprivation; and (c) that the defendant’s enrichment and the plaintiff’s corresponding deprivation occurred in the absence of a juristic reason (Pettkus v. Becker, 1980 CanLII 22 (SCC),  2 S.C.R. 834, at p. 848; Garland, at para. 30; Kerr, at paras. 30-45). While the principled unjust enrichment framework and the categories coexist (Kerr, at paras. 31-32), the parties in this case made submissions only under the principled unjust enrichment framework. These reasons proceed on this basis.
 This principled approach to unjust enrichment is a flexible one that allows courts to identify circumstances where justice and fairness require one party to restore a benefit to another. Recovery is therefore not restricted to cases that fit within the categories under which the retention of a conferred benefit was traditionally considered unjust (Kerr, at para. 32). As observed by McLachlin J. in Peel (at p. 788):
The tri‑partite principle of general application which this Court has recognized as the basis of the cause of action for unjust enrichment is thus seen to have grown out of the traditional categories of recovery. It is informed by them. It is capable, however, of going beyond them, allowing the law to develop in a flexible way as required to meet changing perceptions of justice.