This Understanding treatise is designed for first-year law students enrolled in Contracts. The text explains common law principles of contract law using cases and examples that students commonly encounter in this first-year course. It also explains and illustrates Article 2 of the Uniform Commercial Code, which deal with sales of goods that are frequently covered in Contracts. This edition includes material on the United Nations Convention on the International Sale of Goods, and the UNIDROIT Principles of International Commercial Contracts, appropriate to a basic course in Contracts, and on modern statutes regarding electronic contracting. The fourth edition contains a revised chapter on the difficult concept of Promissory Estoppel and includes discussions of recent decisions dealing with consumer arbitration clauses. Citations to journal articles have also been updated to reflect recent scholarship on these and other topics.