This report presents the youth Digital Skills Indicator (yDSI), a unique, extensively cross-nationally validated measurement tool with 31 items, distributed over digital skills and digital knowledge questions, that can be used for large-scale population research. The yDSI is the only measurement tool for youth digital skills that has been tested using the full range of validation practices. Over a period of six months, consultation with experts (face validity), cognitive interviews (content validity), pilot surveys (construct validity) and performance tests (criterion validity) with young people were conducted in a wide range of European countries. A review of the literature led to a framework identifying four dimensions that constitute digital skills: (1) technical and operational skills; (2) information navigation and processing skills; (3) communication and interaction skills; and (4) content creation and production skills. Across all four dimensions a distinction should be made between being able to use the functionalities of information and communication technologies (ICTs) (functional aspects) and understanding why ICTs are designed and content is produced in certain ways and being able to use that knowledge in managing interactions in and with digital spaces (critical aspects). Existing publications that report on survey instruments to measure digital skills, tend to cover technical and operational and information navigation and processing skills more than they do communication and interaction and content creation and production skills. Furthermore, functional aspects are more commonly measured than critical aspects of skills.