OpenStreetMap (OSM) is currently the most extensive and widely-used example of Volunteered Geographical Information (VGI) available on the Internet. The aim of the OSM project is to provide a free and openly accessible spatial database. The data is provided by volunteers, who collect and contribute it to the OSM database using a variety of techniques and methods. OSM data is then most commonly used and accessed via a user-friendly webbased map on www.openstreetmap.org. The spatial data corresponding to any OSM mapped area can be exported in a special XML based format, namely OSM-XML. This provides a convenient and dedicated transport format which matches the OSM databases' model. Using these OSM-XML files one should be able to extract information about contribution patterns and tagging summaries for the data. However, the simplicity of OSM-XML is also potentially its greatest disadvantage. Processing OSM-XML data files efficiently can be problematic given that mapped areas can produce complex, large files. In this thesis we present the design and implementation of a new Java-based software application called the OpenStreetMap Contributor Analysis Tool (OSM-CAT) for computing contribution summaries from OSM-XML. OSM-CAT allows users to process OSM-XML data efficiently, and automatically produces a detailed summary of the contents of the dataset. This analysis places specific emphasis on 'interesting' statistics, such as who contributed to the OSM data in a chosen area, what types of contributions were made to the OSM data, when these contributions were made, and the accuracy of map feature tagging. While similar tools exist that do some of these tasks, OSM-CAT provides GIS researchers and interested individuals with a complete and integrated overview of contributions to OSM, corresponding to the input OSM-XML datasets. We present a full analysis of OSM-CAT on a large set of OSMXML datasets, and discuss its usefulness to the OSM community and beyond. We close the thesis with some conclusions, and set out a number of issues for consideration as future work. A comprehensive appendix is provided with additional information for those wishing to use OSM-CAT.