A meaningful comparison of paediatric cancer outcomes over time or by geographical location requires a standardised and systematic collection of data on incident cases, survival outcomes, and clinical stage. For incidence and survival outcomes, standard variables have been defined, and guidance for reporting completeness, timeliness, and accuracy of data are available. However, guidelines for reporting clinical stage, or size of the tumour and extent of spread, have historically been less straightforward for paediatric patients. For adult cancers, most of which are carcinomas, the tumour, node, and metastasis staging system consisting of assessments of tumour size and invasiveness, disease spread to nearby lymph nodes, and presence of metastatic disease is widely used and considered to be adequate. By contrast, few childhood cancers are carcinomas—most are leukaemias, lymphomas, sarcomas, and other histological subtypes. Therefore, several disease-specific staging methods for children have evolved over time, often incorporating practical prognostic or risk assignment concepts (eg, the PRETEXT staging system for hepatoblastoma) or resulting in diverging systems based on cooperative group practices, as is the case for staging Wilms' tumour. To address the issue of multiple staging systems for the purposes of paediatric cancer registration, the Toronto Childhood Cancer Stage Guidelines—a consensus-based harmonisation of principles and guidelines now endorsed by the Union for International Cancer Control—were compiled in 2016 and sought to standardise paediatric cancer stage across 16 childhood cancers.