The subject of research is litigation qualitate qua. Or, more accurately: the legal concept by which a person litigates on behalf of another person or other persons, thereby acting as a procedural party. This form of legal representation has surprisingly many applications. The ‘usual suspects’ are the trustee in bankruptcy, the guardian, and the parent litigating on behalf of the minor. Other applications include the group respresentative filing a class action claim on behalf of consumers, the copyright collecting agency litigating on behalf of the associated authors, the reciprocal mandate that the law bestows on joint proprietors and married couples as to the community property, some types of third party funding, … The Belgian doctrine tends to focus solely on the specific applications of this legal concept, bypassing the concept as such. This fragmented approach is not without risk. There is indeed a real threat of contradictions and inconsistencies, which in turn undermines legal certainty and transparency. Moreover, there seems to be doubt about the legal rules governing this form of litigation. Quite often, doctrine and case law refer to the substantive law rules on agency. However, these rules are not always easily applicable to the specific context of legal proceedings. The central aim of this research is to transcend the particular applications and to work out some degree of coherence. To what extent can a general procedural framework be created that encompasses the many manifestations of litigation qualitate qua? And how would that procedural framework relate to the general (substantive) rules on agency?