Non-generally accepted accounting principles (GAAP) (pro forma) earnings form an increasingly important part of firms' performance reporting narrative. This paper reviews the academic and professional debate surrounding non-GAAP earnings reporting by management. I argue that the demand for customised performance reporting is a natural response to constraints imposed by a one-size-fits-all reporting system and that the non-GAAP phenomenon forms part of a long-standing debate over the definition and presentation of periodic performance. A review of extant research suggests non-GAAP disclosures are driven by informative reporting and opportunistic motives. Opaque presentation of non-GAAP earnings is associated with earnings mispricing, particularly among unsophisticated investor groups. Regulations and governance systems designed to ensure transparency are associated with higher quality disclosures and less mispricing. While customised reporting behaviour is evident in many settings, I argue that such disclosures create particular risks in a financial reporting context because they threaten the integrity of the underlying reporting system. Prevailing regulatory approaches are reviewed and factors limiting disclosure transparency are highlighted. The paper concludes with suggestions for further research.