The European opposition system provides a central inter partes mechanism for revoking, or limiting, European patents in all designated states after grant. About 6-7% of European patents are opposed, but the proportion varies between different technologies.
Oppositions involve a relatively fresh look at the granted patent, normally by different examiners. An unsuccessful opposition does not normally close off the possibility of a further attack in each designated state, but a successful opposition is final and cannot be overturned in the designated states.
After filing, the opposition enters a written phase in which amendments and documents are filed, which is (usually) followed by a hearing at which the decision is taken. Opposed patents are maintained, revoked, or amended; each outcome occurs in about 33% of cases.
The decision may be appealed to an EPO Board of Appeal, and the appeal follows roughly the same process as in the first instance.