Even though the type of software-intensive systems has transitioned from the mainframe to the client-server and from the client-server to the cloud, usability is one of the significant quality characteristics throughout the transitions. To achieve and improve usability, it is important to ensure a common understanding of usability concepts and to utilize proven usability evaluation technologies. Quality characteristics and subcharacteristics of software-intensive systems, including usability related ones, are defined in the quality models, i.e. product quality model and quality in use model, of ISO/IEC 25010:2011 in ISO/IEC 250nn SQuaRE (Systems and Software Quality Requirements and Evaluation) series. The SQuaRE series also defines quality measures for quality characteristics and subcharacteristics to be used for quality evaluation. The quality models of software-intensive systems are now under revision considering evolved systems and software technologies and influence on various stakeholders, including “Public”. This paper describes the international standardization of usability definition and evaluation from the developers’ point of view. First, the overview of the SQuaRE is introduced. Second, concepts and technologies related to usability in the SQuaRE are explained. Third, the study results of the application of the SQuaRE to actual projects are introduced, focusing on usability. Finally, future perspectives of international standardization of usability of software-intensive systems are described.