Action C21 of the European programme for Cooperation in the field of Scientific and Technical Research (COST - http://www.cost.esf.org/) is dedicated to investigating urban ontologies for an improved communication in urban civil engineering projects. The Action, known informally as "Towntology", brings together a large and heterogeneous grouping from across Europe, whose interests range from construction to urban tourism and from transport infrastructure to resource visualisation. On 6-7 November 2006, in Geneva, the Action convened a successful workshop to address emerging issues in the field. This volume presents the contributions to that workshop, in many cases revised afterwards to capture some of the outcomes of discussion. Many of these contributions are from members of the Towntology group, but there are also contributions from other European researchers, and from researchers in the US. The volume represents a valuable overview of major current issues in the field of urban ontologies and encapsulates many useful and different approaches. We hope that it will serve not only as a worthy outcome of Action C21, but also as a valuable resource for a wide range of researchers.