Art. 83 EPC stipulates that the application "shall" (previously "must") disclose the invention in a manner sufficiently clear and complete for it to be carried out by a person skilled in the art. The related ground for opposition is in Art. 100(b) EPC.
The subject-matter of an application must be sufficiently disclosed at the date of the application (see in this chapter II.C.2.), based on the application as a whole (II.C.3.1.), including examples (II.C.5.3.), and taking into account the common general knowledge of the skilled person (II.C.4.). At least one way of enabling the person skilled in the art to carry out the invention must be disclosed (II.C.5.2.), but this is sufficient only if it allows the invention to be performed in the whole range claimed (II.C.5.4.). Parameters must be sufficiently defined (II.C.5.5.). The disclosure must also be reproducible without undue burden (II.C.6.); evidence for this from post-published documents is allowable under certain circumstances (II.C.6.8.).
The distinction between the requirements for sufficiency of disclosure under Art. 83 EPC and clarity of the claims under Art. 84 EPC (II.C.7) is important, as examination in respect of the requirements of Art. 83 EPC is still permitted during opposition proceedings, whereas in respect of Art. 84 EPC it is limited to cases where there has been an amendment (see also G 3/14 in chapter II.A.1.4.).
The burden of proof generally lies with the opponent to establish that an invention is insufficiently disclosed (II.C.9.).