This volume serves as the conference proceedings for the Third International ConferenceonGeoSensorNetworks(GSN 2009)that washeld in Oxford,UK, in July 2009. The conference addressed issues related to the deployment of geos- sor networks and the collection and management of real-time geospatial data. This volume includes papers covering a variety of topics, ranging from sensing, routing and in-network processing, to data modelling, analysis, and appli- tions. It re?ects the cross-disciplinary nature of geosensor networks by bringing together ideas from di?erent ?elds, such as geographic information systems, d- tributed systems, wireless networks, distributed databases, and data mining. The papersincludedinthis volumepushthe frontiersofGSN researchins- eral new directions. First, they shift the traditional paradigm of ?xed resour- constrained sensor networks to more versatile scenarios involving mobile sensors with varying levels of computation, sensing, and communication capabilities. Second, emerging applications, such as urban sensing, are raising interesting new GSN challenges, such as participatory sensing, as well as the need to st- dardizeprotocolsandinterfaces,andintegratemultiple sensornetworks.Finally, technological advances in geosensor networks have enabled the collection of vast amounts of real-time high-resolution spatio-temporal data; the sheer volume of these data raises the need for e?cient data modelling, analysis, and pattern extraction techniques.