The long-awaited Supreme Court ruling in Guest v Guest has provided a welcomed judgment on a core principle in equity and trusts law, proprietary estoppel. The leading judgment not only provides guidance on the purpose of relief for proprietary estoppel, but also provides much needed clarity on a persistently debated issue in case law, namely whether an ‘expectation-based’ or ‘reliance-based’ approach to calculating relief for proprietary estoppel should be adopted, with the judgment providing an insightful step-by-step guide for the exercise of judicial discretion in such cases to ensure appropriate and justified relief is awarded.