Intervention is an historical mix of the common law of amicus curiae ('friend of the court'), and - more recently - statute law (R13 of the Rules of Civil Procedure). It is integrally related to the law of standing (which see) and you can think of it as a form of 'standing-lite', where the intervenor (once that status is granted by the court) has a limited and order-specified role in the case. The potential rights and role of intervenors exclude both the ability to commence proceedings, and the right to raise new legal issues and remedies - but otherwise intervenors can have a huge role in litigation.
