Authoritative treatise addressing the tax classification and tax treatment of perpetual debt instruments and super-maturity debt instruments. In order to analyse the tax aspects of perpetual debt and super-maturity debt, the background regarding commercial, rating, regulatory and accounting issues is provided. Although the main focus of the article is on the tax classification of perpetual debt and super-maturity debt under domestic law, EU corporate law directives and income tax treaties; the treatment under domestic law is illustrated by US, German and Danish law. More specifically, the article considers the economic and legal framework, followed by a comparative overview of tax law classification in the United States and Germany and gives an analysis of the tax classification and treatment under Danish tax law. Next, the article analyses whether payments under perpetual debt and super-maturity debt instruments fall under the scope of the EU corporate tax directives, and also considers the income tax treaty protection of payments under perpetual and super-maturity instruments.