Contemporary Web 2.0 technologies facilitate the establishment of large online communities of mass collaboration. In shared workspace environments, millions of people interact without knowing each other. The outcome is openly accessible and constantly developing collective knowledge in the form of a more or less organized knowledge base of digital artifacts. With this dissertation I advance a differentiated approach for studying and understanding the principles that underlie knowledge

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... t under these conditions. The work builds on a theoretical consideration of collaborative learning and knowledge building stemming from the interdisciplinary learning sciences and research on computer-supported collaborative learning (CSCL) in particular. Knowledge is understood as substance with static structure that changes over longer periods of time through the activity of community participants in analogy to the progress of scientific ideas in different domains. A complex systems perspective is used to explain knowledge as an emergent phenomenon that amounts to more than the additive collection of individual contributions. This macro level of processes and structures in a community determines to a large extent how new contributions are made and thus how knowledge develops. Based on these conceptualizations, the present dissertation empirically examines large real-life data sets from the online communities Wikipedia and Wikiversity. Knowledge is captured as a network of interconnected articles in different knowledge domains. The topological position of the articles in the networks is evaluated through established network analysis metrics in order to indentify pivotal articles that form the static structural backbone of the collective knowledge. A cross-sectional analysis demonstrates that pivotal articles tend to be written by authors with extensive contribution experience in the community. In a longitudinal study, a mechanism of knowledge development is evidenced according to which pivotal articles attract new knowled [...]