Abstract OBJECTIVE Core Outcome Sets (COS) define the minimum outcomes that should be measured and reported in all clinical trials for a specific health condition or health area. The aim was to develop two COS for intracranial meningioma; one for clinical trials (COSMIC: Intervention) and one for observational studies (COSMIC: Observation). METHODS A study advisory group was formed with representation from international stakeholders of charities (IBTA, TBTC, brainstust), neurosurgery societies (SBNS, BIMS) and neuro-oncology societies (BNOS, EANO, EORTC, ICOM, RANO-PRO, SNO). Two systematic literature reviews and trial registry searches were performed to identify outcomes reported in published and ongoing 1) meningioma clinical trials, and 2) studies of incidental/untreated meningioma. Outcomes were deduplicated and categorised. Individuals were recruited from stakeholder groups (including meningioma patients and carers, healthcare professionals, researchers) to participate in 2-round, international, modified eDelphi surveys. The final core outcome sets were ratified through online consensus meetings. RESULTS The COSMIC: Intervention systematic review identified 660 verbatim outcomes, rationalized into 24 Delphi survey items. Following the consensus meeting, 15 outcomes were included in the final set. For COSMIC: Observation, 267 verbatim outcomes were identified and rationalized into 17 Delphi survey items. 16 outcomes were included in the final set. There were eight core outcomes common to both COSMIC: Intervention and COSMIC: Observation COS, namely tumour growth, physical, emotional, and neurocognitive functioning, overall quality of life, progression-free survival, meningioma specific mortality and overall survival. Role and social functioning were core outcomes in COSMIC Observation but not COSMIC intervention. CONCLUSIONS The COSMIC study has defined the core outcome sets for use in future meningioma clinical trials and studies. Role and social functioning were important to incidental meningioma patients and likely reflects the anxiety and uncertainty around clinical treatment. International stakeholder agreement should ensure widespread use and adoption.