How do you respond to a patent office action, or its EU equivalent?

In the US, an office action is the examiner's written response to your application, usually raising objections or rejecting some or all claims based on prior art, with 3 months to respond, extendable to 6 with a fee. Missing the deadline causes the application to go abandoned. In the EU, the equivalent is a Communication under Article 94(3) EPC, an examination report from the EPO, with a standard 4 month deadline extendable through further processing; applications typically go through about two rounds before grant. In both systems, the examiner's citations usually point to genuine prior art, so the response is a negotiation over claim scope rather than a fight to overturn the rejection outright. Narrowing claims strategically, rather than ignoring the objection or over narrowing out of caution, is where experienced counsel earns its fee in either jurisdiction.

Sources: USPTO, Office Actions, EPC Article 94, Examination of the European patent application.