Espacenet (formerly often written as esp@cenet) is a free online service for searching patents and patent applications. Espacenet was developed by the European Patent Office (EPO) together with the member states of the European Patent Organisation. Most member states have an Espacenet service in their national language, and access to the EPO's worldwide database, most of which is in English. Early 2011, the Espacenet worldwide service contained records on more than 70 million patent publications.
By launching Espacenet in 1998, the EPO is said to have "revolutionized public access to international patent information, releasing patent data from its paper prisons and changing forever how patents are disseminated, organized, searched, and retrieved."
In 2004, Nancy Lambert considered that, although free, Espacenet, like the United States Patent and Trademark Office (USPTO) database of US patents, "still tend[ed] to have primitive search engines and in some cases rather cumbersome mechanisms to download patents." She reported it as being deliberate, on the part of the USPTO and EPO, "who have said they do not wish to compete unfairly with commercial vendors".
By 2009, Espacenet offered the so-called SmartSearch which allows a query to be composed using a subset of Contextual Query Language (CQL) and in early 2011, the site was completely redesigned with a new look and feel.
Example of search 
On Espacenet's "Advanced Search" page, searching for an application or for a patent by its PCT application number is done in the following way: one enters, in the "Application number" field, firstly, the letters "WO", then the year of the PCT, then the country of origin and finally the five digit identification code for the patent. For example, "PCT/AU99/00872" would be entered "WO1999AU00872".
See also 
- Cooperative Patent Classification
- International Patent Classification
- Patent classification
- Prior art
- esp@cenet is a trademark of the European Patent Organisation (see community trade marks No: 001123876, 000881896, 000881813 on CTM-ONLINE – Trade mark consultation service – Basic).
- The "@" in esp@cenet – an old friend retires, Patent Information News issue 1/2011, March 2011, p. 5.
- Espacenet offers free access to more than 70 million patent documents worldwide, containing information about inventions and technical developments from 1836 to today. – EPO web site, Worldwide patent search
- Michael J. White, Espacenet Europe's Network of Patent Databases, Issues in Science & Technology Librarianship (ISSN 1092-1206), Number 47, Summer 2006.
- Lambert, N. (2004). Internet patent information in the 21st century: A comparison of Delphion, Micropatent, and QPAT, 2004 International Chemical Information Conference & Exhibition, Annecy, France, 17–20 October 2004, pp 1–2.
- Patentinfo News issue 4 2010