»The Quality Agreement in the System of Purchase Warranty Law«: The agreement on quality under the law on the sale of goods occupies a central position within the factual elements of a material defect under the law on the sale of goods. It is regularly the focus of case law by the highest courts and has been influenced by the implementation of the Sale of Goods Directive. The present work acknowledges these influences in detail and integrates the agreement on quality under the law of sale into a coherent concept that provides an appropriate solution to individual questions.