Once notice of an application is given to the respondent State, the possibility arises for persons and non-governmental organisations (other than the applicant), as well as States (other than that against which the application has been lodged) to seek permission to intervene in the procedure. These are known as third-party interveners. They may be given permission to submit written comments, and may, exceptionally, also be authorised to take part in a public hearing, should the Court decide to hold one.

In all cases before a Chamber or the Grand Chamber, the Council of Europe Commissioner for Human Rights may submit written comments and take part in hearings (Article 36 § 3 of the Convention) as may a High Contracting Party one of whose nationals is an applicant (Article 36 § 1).