.0, the "dynamic web," is one of the most effective applications to date of information and communication technologies in the way it revolutionized how individuals interact with information. The term Web 2.0 is largely associated with Tim O'Reilley (Graham, 2005; O'Reilley, 2005), and is a loosely defined concept that represents a variety of technologies. Some of these are development platforms while others are applications used mostly for social computing and interaction. The latter group of technologies, also known as the Social Web (Rheingold, 2000) has arguably been in the core of the World Wide Web from its conception (Porter, 2008). Moreover, the developmental components of Web 2.0 that moved computing beyond desktops via the use of web-based "building blocks" such as web services not only made it possible to realize the true potential of the social web, but to a large extent made it synonymous with Web 2.0. Our use of the term Web 2.0 also treats it as identical to the social web.As such, Web 2.0 is omnipresent in virtually every aspect of our lives from social networking to viral marketing to political campaigning. Masses, who to a large extent used to be mere consumers of content generated by relatively few "authorities," are now in a position to be "content providers." This means a sizeable portion of content in circulation today is generated by "novices" either in the domain of that content or the technology used in its generation and dissemination. This, by itself, made inevitable a paradigm shift in the interaction of users with information technology. Another radical change has been in the way people consume user-generated content and communicate with the "creators" or other users of that content.We issued the call for papers for this special issue in 2010 based on our observation of this strong trend in the development, adoption, and use of Web 2.0 applications, and the relative scarcity of research on Web 2.0 from a Human Computer Interaction (HCI) viewpoint. There are many elements that influence the interaction of people with Web 2.0 applications. In our call for papers, we noted: "It is presumable that with better identification of users, their motivations, and interaction needs, user experience with Web 2.0 will be substantially improved." We also highlighted the "any time any place" nature of HCI in the era of Web 2.0 as many users interact with Web 2.0 applications using mobile devices, thus enabling near real-time information sharing. When we reflect upon these observations today, we see that the trend in the adoption of Web 2.0 applications has continued at a tremendous pace. As an example, as of early 2011 when the submissions to this special issue were due, there were over 600 million Facebook and about 175 million Twitter accounts. In 2010 alone, there was a growth of 200 million and 50 million accounts, respectively. 1 Within the same time frame, the number of blog posts published on one of the most popular blogging platforms, WordPress.com, increased from approximately 1.8 million t...