It is our great pleasure to welcome you to the 11th ACM Conference on Web Science (Websci'19), June 30 - July 3, 2019, Boston, MA, USA. This year, the conference theme is "Synergies for the Good: The Web and Society". We welcomed interdisciplinary contributions, especially those that had a broad perspective on the web, including those that combined analyses of web data with other types of data (e.g., from surveys or interviews) to better understand user behavior (online and offline); carried out longitudinal studies; presented successful cases of interdisciplinary and cross-disciplinary web research; used mixed-method approaches; critically reflected on the methods used; discussed responsible forms of Web Science (e.g. regarding standards, methods, generalizability of results); and/or those that reflected on the societal impact of web research, how the web is perceived in the media and in society, and whether this clashes with our self-image of Web Science. Thus, research on the interaction of society and the web was invited, and we received submissions highlighting web implications, synergies derived, and how the web as a socio-technical system will evolve in future. WebSci'19 was a unique conference where a multitude of disciplines converged in a creative and critical dialogue with the aim of understanding the web and its impacts. WebSci'19 welcomed participation from diverse fields including (but not limited to) art, anthropology, computer and information sciences, communication, economics, humanities, informatics, law, linguistics, philosophy, political science, psychology, and sociology. Following the tradition of earlier conferences, contributions to WebSci'19 aimed to cross traditional disciplinary boundaries. The community engaged with novel and thought-provoking ideas and discussed original research, work in progress, analysis, and practice in the field of Web Science, its current theoretical, methodological, and epistemological challenges as well as Web practices of individuals, collectives, institutions, and platforms. This year we were very pleased to accept 41 submissions for the regular research track chosen out of 130 submissions. We are grateful for the support of the Program Committee which consisted of 16 senior members and 66 regular members who selected an interesting, varied, exciting program comprising 31 long and 10 short papers. In addition to the posters selected from the Call for Posters, eight contributions were invited to be presented as a poster because they sparked fruitful discussions among the reviewers and were deemed to be of great interest and relevance to the WebSci'19 community. This year WebSci'19 encouraged authors to particularly prepare and publish reproducibility information of conducted research and resources, such as source code and datasets. Authors were asked to add (if possible) a link (e.g. DOI or URL) to data or any other information relevant to their submission. With this measure WebSci'19 aimed at raising awareness of the reproducibility issue and demonstrated that, as a community-driven conference, it subscribes to and actively promotes Open Science principles in research.