Ong on Estoppel, by the prolific Professor Denis SK Ong, is one of the very few scholarly book-length treatments of the doctrine of estoppel published in Australia. Topics addressed with Ong’s characteristic meticulous examination of both Australian and English authorities include: estoppel’s origins as a rule of evidence; estoppel by conduct (including representation); estoppel by deed and estoppel by convention; promissory and proprietary estoppel; estoppel and election; estoppel between banker and customer; and estoppel by representation is contrasted with the defence of the change of position. Readers are advised that the book does not deal with issue estoppel. Of particular interest to readers will be the author’s carefully argued view that in Australia at least (and notwithstanding several judgments of the NSW Court of Appeal) proprietary estoppel is merely a sub-species of promissory estoppel and that there is in reality a single overarching doctrine of estoppel. This is Professor Ong’s ninth major treatise in equity law and complements his earlier works which now include: Trusts Law in Australia (now in its 5th edition); Ong on Equity; Ong on Specific Performance; Ong on Subrogation; Ong on Rescission; Ong on Contribution; Ong on Rectification and Ong on Tracing. Ong on Estoppel is an essential reference for commercial lawyers, barristers, academics and the judiciary.