The Uniform Commercial Code (UCC) sets the standards of good faith in a commercial transaction for the sale of goods. With every sales contract, there is an implied obligation for both the seller and the buyer to negotiate the contract and perform under the terms of the contract in good faith. The agreement between both parties and the customs in the industry determine how the good faith standard should be applied to a particular transaction. Generally, the meaning of good faith, though always based on honesty, may vary depending on the specific context in which it is used. A person is said to buy in good faith when he or she holds an honest belief in his or her right or title to the property and has no knowledge or reason to know of any defect in the title. In section 1-201 of the UCC good faith is defined generally as "honesty in fact in the conduct or transaction concerned." Article 2 of the UCC says "good faith in the case of a merchant means honesty in fact and the observance of reasonable commercial standards of fair dealing in the trade." The sales contract will generally determine which party is required to perform first. This provision helps to determine if the buyer or the seller is in breach of the agreement due to failing to perform as stated by the contract. Either the seller must deliver the items before the buyer is required to accept and pay or the buyer must pay for the items before the seller has the duty to act in good faith and deliver the items in a reasonable manner. If the contract does not specifically define who is required to perform, industry customs and fair trade may determine what is acceptable for the transaction. Under the UCC, the buyer is required to pay for the goods when they are delivered, unless the contract states otherwise. Therefore, the UCC imposes an obligation of good faith on the performance of every contract or duty under its purview. The law also generally requires good faith of fiduciaries and agents acting on behalf of their principals. This article discusses problems of the principles of good faith under the UCC. Specifically, this paper focuses on the interpretation of UCC sections and analysis of various cases. By comparing, also, UCC and Korean law, the paper proposes some implications of good faith issues for Korean law.