ISO9126-1 represents the latest (and ongoing) research into characterizing software for the purposes of software quality control, software quality assurance and software process improvement (SPI). This article defines the characteristics identified by ISO 9126-1. The other parts of ISO 9126, concerning metrics or measurements for these characteristics, are essential for SQC, SQA and SPI but the main concern of this article is the definition of the basic ISO 9126 Quality Model.