The chapter on online data quality starts with a condensation of approaches to presenting data quality. The intuitive approach is followed by an empirical approach and then by a systems view that treats data quality as a theory-based ontological concept. A trade-off situation arises as the more rigid developments lack the very central pragmatic quality concept of 'ﬁtness for use'. The investigation into the online environment exemplifies the effects on traditional data of the new medium. Secondly, the online oriented development of metadata standards introduces the data quality dimension 'documentality' as accumulations of datasets in archives have requested a continuous development of high-level documentation. Furthermore, new online data have arisen as non-reactive Internet generated data in the form of accurate traces of behaviour are far from incomplete traditional inquiries into respondents' recollections of behaviour. The actual browsing on the Internet, the content of websites, and social facts such as public tweets are the foundations of research of the online world. Finally, the Internet has positively impacted the possibility of performing experiments. The online environment has improved the data quality and data are of more widespread methodological origin, data are more plentiful, and data have generally become more useful for research.