The aim of treating childhood cancer remains to cure all. As survival rates improve, long-term health outcomes increasingly define quality of care. The International Childhood Cancer Outcome Project developed a set of core outcomes for most types of childhood cancers involving relevant international stakeholders (survivors; pediatric oncologists; other medical, nursing or paramedical care providers; and psychosocial or neurocognitive care providers) to allow outcome-based evaluation of childhood cancer care. A survey among healthcare providers (n = 87) and online focus groups of survivors (n = 22) resulted in unique candidate outcome lists for 17 types of childhood cancer (five hematological malignancies, four central nervous system tumors and eight solid tumors). In a two-round Delphi survey, 435 healthcare providers from 68 institutions internationally (response rates for round 1, 70–97%; round 2, 65–92%) contributed to the selection of four to eight physical core outcomes (for example, heart failure, subfertility and subsequent neoplasms) and three aspects of quality of life (physical, psychosocial and neurocognitive) per pediatric cancer subtype. Measurement instruments for the core outcomes consist of medical record abstraction, questionnaires and linkage with existing registries. This International Childhood Cancer Core Outcome Set represents outcomes of value to patients, survivors and healthcare providers and can be used to measure institutional progress and benchmark against peers.