Unjust enrichment, along with such subjects as contract and tort, is one of the law's primary sources of rights and obligations. This course examines unjust enrichment, and the law's response to unjust enrichment, called restitution. Liability in unjust enrichment is encapsulated in the phrase "unjust enrichment at the expense of the plaintiff". We commence with enrichment. Not all benefits received by the defendant are enriching and the courts have developed tests to determine whether the defendant is enriched and whether this enrichment is at the plaintiff's expense. The next question is injustice, this question being answered by the unjust factors. We will cover various unjust factors, including mistake, failure of basis, pressure and policy motivated unjust factors. Finally, we will look at two defences: change of position and estoppel.