Instead of, or in addition to, filing a national patent application at a national patent office (eg a GB patent application at the UK Intellectual Property Office (UK IPO)), or a regional patent application at a regional patent office (eg a European patent application at the European Patent Office (EPO)), an international patent application can be filed under the Patent Cooperation Treaty (PCT). This Practice Note provides an overview of the main provisions of the PCT and the process for filing an international patent application. For more information about applying for a GB patent at the UK IPO, see Practice Note: Patent applications—how to obtain patent protection in the UK. For a list of the regional patent offices, see here and for more information about European patent applications at the EPO, see Practice Note: European patent—application procedure. The Patent Cooperation Treaty (PCT) The PCT, which was established in 1970, assists applicants in seeking patent protection internationally for their inventions. The PCT has been ratified by over 150 countries, including the UK. For a list of