The notice of opposition must also state the grounds on which the opposition is based (R. 76(2)(c) EPC; R. 55(c) EPC 1973). The grounds for opposition which can form the legal basis of an opposition are contained in an exhaustive list in Art. 100 EPC. Accordingly, an opposition may only be filed on the grounds that (a) the subject-matter of the European patent is not patentable, (b) the European patent does not sufficiently disclose the invention or (c) the subject-matter of the European patent goes beyond what was originally disclosed. Each ground for opposition listed in Art. 100 EPC corresponds to a requirement for grant laid down elsewhere in the EPC.

The Enlarged Board of Appeal set out a number of fundamental principles in G 1/95 (OJ 1996, 615):

Whereas the grounds for opposition in Art. 100(b) and (c) EPC 1973 each relate to a single, separate and clearly delimited legal basis on which an opposition can be based, i.e. insufficient disclosure or unallowable amendment before grant, the same does not apply to Art. 100(a) EPC 1973. Art. 100(a) EPC 1973 not only refers to the general definition of patentable inventions under Art. 52(1) EPC 1973 and the exceptions to patentability under Art. 53 EPC 1973; it also refers to a number of definitions given in Art. 52(2) to (4) EPC 1973 and Art. 54 to 57 EPC 1973 for the terms "invention", "novelty", "inventive step" and "industrial application", which, when used in conjunction with Art. 52(1) EPC 1973, set specific requirements and therefore constitute separate grounds for opposition. Taken together, Art. 52 to 57 EPC 1973 therefore do not, for the purposes of Art. 100(a) EPC 1973, constitute a single objection to the maintenance of the patent, but a collection of different objections. Some of these are completely independent of each other (e.g. Art. 53 EPC 1973 and Art. 52(1) EPC 1973, Art. 54 EPC 1973) whereas others might be more closely related (e.g. Art. 52(1) EPC 1973 and Art. 54 EPC 1973; Art. 52(1) EPC 1973 and Art. 56 EPC 1973). For an opposition to be admissible under Art. 100(a) EPC 1973, it has to be based on at least one of the grounds for opposition set out in Art. 52 to 57 EPC 1973.

According to the reasoning of G 1/95, one function of R. 55 and 56 EPC 1973 (R. 76 and 77 EPC) is to establish what the notice of opposition has to contain in order to be admissible. R. 76(2)(c) EPC specifies that the notice of opposition has to contain a statement of the extent to which the European patent was opposed and of the grounds (i.e. the legal reasons mentioned above) on which the opposition is based and an indication of the facts, evidence and arguments presented in support of these grounds, i.e. the substantiation. The wording of (c) shows clearly the distinction made between the grounds, meaning the legal reasons or legal bases such as those under Art. 100(a) EPC and the substantiation. Accordingly, in the context of Art. 99 and 100 EPC and of R. 76(2)(c) EPC, a "ground for opposition" has to be interpreted as meaning an individual legal basis for an objection to the maintenance of a patent. It follows in particular that Art. 100(a) EPC contained a collection of different legal objections (i.e. legal bases), or different grounds for opposition, and is not directed to a single ground for opposition.

Non-compliance with Art. 84 EPC is not a ground for opposition, but is a matter to be considered in the context of Art. 101(3) EPC (see also G 3/14, OJ 2015, A102 and in this chapter IV.C.5.2.).

In T 600/08 the board observed that the EPC did not provide for reviewing, as part of opposition proceedings, the legality of a decision by the Receiving Section to correct an applicant's name in a divisional application. The grounds for opposition were set out exhaustively in Art. 100 EPC. Objections to the text of a patent as granted could not be raised on other grounds. It made no difference that the text of the patent in suit had been amended.