With the increasing amount of geographic information available on the Internet, searching, browsing, and organizing such information has become a major challenge within the field of Geographic Information Science (GIScience). As all information is ultimately for and from human beings, the methodologies applied to retrieve and organize this information should correlate with human similarity judgments. Semantic similarity measurement, which originated in psychology, is a methodology fulfilling this requirement and supporting geographic information retrieval. The following special issue presents work on semantic similarity measurement from different perspectives, including cognitive science, information retrieval, and ontology engineering, with a focus on applications in GIScience. It originated in the Workshop on Semantic Similarity Measurement and Geospatial Applications held in conjunction with COSIT 2007, the International Conference on Spatial Information Theory (http://www.cosit.info/). A substantial part of the workshop contributions addressed the need for similarity measurement in geographic information retrieval, including applications in web service discovery, knowledge management, and emergency scenarios. The call for papers to this issue was based on these workshop contributions, discussions, and ∗Final draft, the definitive version is available at www.blackwell-synergy.com.