The legal regulations regarding the joinder of parties in the Financial Court Order and the Tax Code are the focus of this work. They are systematically presented based on case law and procedural literature, with particular emphasis on the rulings of the Federal Fiscal Court. It becomes evident that the constitutional issues involved, especially those related to fundamental rights, are of significant importance for the parties involved in the proceedings and the substantive judgment. This is illustrated, among other things, by comparing the regulations on joinder in social court and administrative court procedures: The particular complexity of third-party involvement in tax litigation provides grounds for a critical constitutional assessment and a legal evaluation of the statutory design of joinder in tax procedural law.