Intervenor is one who voluntarily enters a pending lawsuit because of a personal stake in it, i.e. who has a legal interest that one of the parties succeeds. Such legal interest needs to be present, not a future one. It exists every time the finality of a judgment affects the intervenor. Existence of a legal interest does not necessarily authorize one to intervene in a lawsuit, since there are certain litigations where intervention of a third party is impermissible. Reasons for one’s intervention are various and they have changed over time. However, the most important reason existed permanently and it always comprised mutual interest of the intervenor and the party to litigation to succeed in the lawsuit. Intervenor is one who always stands outside the partisan relationship. Such individual may not file for a lawsuit, nor may he dispose of it. Such right has been reserved exclusively for the parties. Additionally, intervenor may not be a representative to one of the parties. Intervenor may usually enter a pending lawsuit throughout the proceeding, until its finality. One must apply for intervention by a written brief. Since the application to intervene is not an initial action, it may not cause procedural or material legal effects related to filing for a lawsuit and serving of the claim upon the defendant. Litigation produces effects towards the intervenor as of the moment when he was authorized to intervene. During the course of the proceeding intervenor may undertake only such actions which are favorable to the party on whose side he intervened, or to which such party consents, or which the party omitted to undertake. Only the intervenor characterized as joint litigator may undertake the actions opposed to by a party. This is possible, for the finality of the judgment will be applicable to the intervenor as well. If a party joined by an intervenor fails in her claim, it may initiate another lawsuit against the intervenor. In a situation like that, the judgment rendered in the preceding lawsuit shall have an intervention effect. Such effect is broader than finality of the judgment and it may be refuted by the regular objection of poor litigating. Apart from the ordinary, there is also the so-called major intervention. Here a third party files for a lawsuit against both the claimant and the defendant of an already pending lawsuit. The third party is an independent party, while the parties to the initial lawsuit are passive co-litigants. In this case, there is actually no real intervention, but the pending lawsuit is merely a motive for the third party to institute a legal action. There are also special kinds of intervention to litigation, where the intervenor appears as either a guardian of general social interests (intervention of a public prosecutor), or as a guardian of interests of minor children lacking parental care, or of adults who are unable to protect their own interests due to their physical or mental condition (intervention of the Institution of Guardianship).