How can I find out if a particular patent or patent application is still valid?
Legal status information (i.e. where and whether a patent is valid) for European patent and Euro-PCT applications and European patents with unitary effect is available via the EPO's free online European Patent Register, where you can also access the publicly available parts of the application file, including proceedings during the grant opposition, appeal and post-grant (national phase) phases.
Once a patent application is granted, it becomes either a European patent with unitary effect or a conventional European patent that is reverted to the national patent offices of the designated states. If there is a European patent with unitary effect, you can inspect the post-grant proceedings and legal status in the Unitary Patent Register of the Register. If, however, it is a granted conventional European patent, you can inspect the proceedings in EP Legal status. For legal certainty, you should nevertheless contact the relevant national office or inspect the registers of these offices directly using the deep links provided in the European Patent Register (where available).
Legal status information can also be accessed via the Legal events tab in Espacenet. This information is based on EP data and data the EPO receives from the national patent offices worldwide.