A new generation of tools supports the integration of Web 2.0 and semantic Web approaches. Some of these tools-such as Semantic MediaWiki (http://meta.wikimedia. org/wiki/SemanticMediaWiki), BOWiki, (http://onto.eva. mpg.de/bowiki), and Platypus Wiki (http://platypuswiki. sourceforge.net)-provide wiki extensions for creating semantic links between pages. Other tools let users organize tags in some semantic structure, and fully fledged ontology editors such as pOWL support the distributed and collaborative development of ontologies. Commercial tools such as Freebase (http://freebase.com) are also entering the field. Most of these tools are in early development-the collaborative-knowledge-construction field is in its infancy. Few, if any, user studies outline what users expect from such tools and what does or doesn't work. So, we organized the collaborative knowledge construction challenge. The CKC challenge let users try different tools and provide feedback to help us assess the state of the art.