This book investigates the increasingly pervasive use of non-GAAP disclosure in corporate reporting, with a particular focus on the European context under IFRS. It explores both empirical and institutional dimensions, reviewing two decades of academic debate and evaluating recent and upcoming regulatory changes. The book includes a definitional review, an investigation of the most-used non-GAAP measures in Europe, an exploratory multi-country empirical analysis of the informativeness of non-GAAP earnings, and an assessment of compliance with ESMA guidelines.