This chapter introduces the concept and scope of non-GAAP performance indicators within the broader category of voluntary corporate disclosure. It explores the role these indicators play in complementing standard GAAP metrics and addresses the motivations behind their disclosure. The chapter provides a deep discussion on definitional challenges of non-GAAP measures. It also discusses the boundaries of the non-GAAP category, distinguishing it from Key Performance Indicators (KPIs) and GAAP items. A comprehensive definition of non-GAAP performance indicators is developed, taking into account their defining attributes and the context in which they are used. Finally, empirical evidence on the diffusion of non-GAAP measures among European listed firms is presented and discussed.