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Welcome to Hypertext 2009, the 20th ACM Conference on Hypertext and Hypermedia. The conference continues in its tradition of a diverse and multidisciplinary approach to the study of the "link" in all its manifestations. Links between any types of objects, from documents and media to people, are at the center of Hypertext from many perspectives: their design, management, applications, semantics, presentation, dynamics, effects on society, and the knowledge that can be derived from their analysis.

Hypertext 2009 also continues to be organized around tracks autonomously organized with separate chairs and program committees. This year we have three tracks in the technical program.

The Information Structure and Presentation track represents a multitude of topics, which were traditionally represented at ACM Hypertext Conferences. The track program targets formal study of scholarly, structural, sculptural, spatial, open, dynamic and adaptive or any other type of hypertext (or Web-based information system). This track also focuses on how hypertext approaches and technologies can be applied to structure and present information in diverse domains, and how hypertext techniques can be exploited in classical and advanced applications.

The People, Resources, and Annotations track explores social annotations, which have rapidly risen as one of the most exciting recent developments in Web science. Users can easily markup other authors' resources via collaborative mechanisms such as tagging, filtering, voting, editing, classification, and rating. These social processes lead to the emergence of many types of links between texts, users, concepts, pages, articles, photos, videos, tags, and so on. The track's focus is on design, analysis, and modeling of information systems driven by social linking.

Finally, the Hypertext and Community track examines and reflects upon social cyberculture in electronic media, ranging from literary fiction and creative scholarship to blog and microblog networks, social sites, games, auctions, net art, and markets.

The Hypertext 2009 technical program received 117 submissions, a 70% increase from the previous year, despite the global economic downturn. These submissions originated from 36 countries in Europe, Asia, Africa, Australia, North and South America. 37 papers (26 long and 11 short) were selected for presentation at the conference, yielding an acceptance rate of 31.6%. We feel that the papers contained in these proceedings represent a strong, diverse, and exciting program.

We are fortunate to feature two stellar keynotes by Lada Adamic of the University of Michigan on The Social Hyperlink and by Ricardo Baeza-Yates of Yahoo! Research on Relating Content by Web Usage. In addition, the program includes a poster & demo session (also in these proceedings), an ACM student research competition, and four workshops:

• Web 3.0: Merging Semantic Web and Social Web organized by Federica Cena (University of Torino, Italy), Pasquale Lops (University of Bari, Italy), and Rosta Farzan (University of Pittsburgh, USA).

• Dynamic and Adaptive Hypertext: Generic Frameworks, Approaches and Techniques organized by Paul De Bra and Mykola Pechenizkiy (Eindhoven University of Technology, the Netherlands).

• New Forms of Xanalogical Storage and Function organized by Fabio Vitali and Angelo Di Iorio (University of Bologna, Italy), and Jamie Blustein (Dalhousie University, Canada).

• Tagging Dynamics in Online Communities organized by Vittorio Loreto and Andrea Capocci (Sapienza University of Rome, Italy).

Finally, Hypertext 2009 attendees have a chance to experiment with applications mixing real-world data and on-line data. Active RFID tags in the badges of volunteers track the real-time relations of physical proximity between the attendees. The data collection and visualization systems is provided by the SocioPatterns.org project and exposes API methods that allow developers to mash up real-world links between the attendees with other types of linking information from the Web.

We are proud that Hypertext 2009 takes place in Torino (Turin). The capital of the Piedmont region in northwestern Italy, Torino lies at the foot of the Alps, the majestic mountains that hosted the 2006 Winter Olympics. First capital of the Kingdom of Italy, then one of the European centers of baroque, today Torino is a dynamic city known for its industry (Fiat and Lancia car makers are headquartered here); art and culture (its 40+ museums include the world's largest Egyptian collection outside of Cairo or the National Museum of Cinema located inside the Mole Antonelliana); sports (home of Juventus FC and Torino FC); research and education (including the University of Torino, the Polytechnic, and the Institute for Scientific Interchange Foundation); and cuisine (solid chocolate was born here in the 18th century). We hope that all attendees have an opportunity to enjoy the many cultural, artistic, historic, architectural, and culinary pleasures that Torino has to offer.