Intellectual property and the patent system in particular have garnered a lot of attention, even in the public media, over the last few years. This monograph is not concerned with any of the controversial issues regarding the patent system itself but it does examine a very real and growing problem: searching for innovation. The target collection for this task does not consist of patent documents only, but it is in these documents that the main difference is found compared to web or news information retrieval. In addition, the issue of patent search implies a particular user model and search process model. Patent Retrieval addresses the question of how research and technology in the field of Information Retrieval assists or even changes the processes of patent search. It is a survey of work done on patent data in relation to Information Retrieval in the last 20 to 25 years. It explains the sources of difficulty and the existing document processing and retrieval methods of the domain, and provides a motivation for further research in the area. Patent Retrieval is an ideal reference for Information Retrieval researchers interested in the patent domain and for patent information professionals.