This publication gives an account of the Public Annotation of Cultural Heritage research project (PACE) conducted at the Crossmedialab. The project was carried out between 1 January 2008 and 31 December 2009, and was funded by the Ministry of Education, Culture, and Science. Three members of the Dutch Association of Science Centres (Vereniging Science Centra) actively participated in the execution of the project: the Utrecht University Museum, the National Museum of Natural History (Naturalis), and Museon. In addition, two more knowledge institutes participated: Novay and the HU University of Applied Sciences Utrecht. BMC Consultancy and Manage¬ment also took part in the project. This broad consortium has enabled us to base the project on both knowledge and experience from a practical and scientific perspective. The purpose of the PACE project was to examine the ways in which social tagging could be deployed as a tool to enrich collections, improve their acces¬sibility and to increase visitor group involvement. The museums’ guiding question for the project was: ‘When is it useful to deploy social tagging as a tool for the benefit of museums and what kind of effect can be expected from such deployment?’ For the Crossmedialab the PACE project presented a unique opportunity to conduct concrete research into the highly interesting phenomenon of social tagging with parties and experts in the field.